What is the calculation for placement of gas spring shocks so the door and shock aligne I am trying it install these shocks onto a 2 foot tall by 4 foot wide door. I have a 7 inch gas shock, but were I installed the shock when I close the door it only closes half way and the shock is totally callapsed???

No need for a wheel alignement you didnot change the hight of the car

Bilsteins are about the best shocks you can buy.

With those miles (150000) on the van I would have also replaced all 4 springs and all 4 shocks., It is stupid to put new shocks on the front and leave worn out shocks on the rear . - The rear shock absorbers should also be replaced with new items.
(Springs are inexpensive to replace so I am surprised you did not renew them because it should be done. The old ones will have sagged significantly by now.)

On each end of the shock.theres a clip .small flat head .pop the clip up and pull shock off reverse procedure to install ...easy with two people or a chunk of 2/4 to hold door up

REMOVE REAR SEAT AND PACKAGE SHELF TO GAIN ACCESS TO THE UPPER SHOCK ATTACHING NUTS.REMOVE UPPER ATTACHING NUTS. RAISE VECHICLE REMOVE REAR WHEEL AND SUPPORT THE LOWER CONTROL ARM WITH A JACK REMOVE LOWER SHOCK ABSORBER MOUNTING BOLT. REMOVE SHOCK ABSORBER ASSEMBLY FROM THE VECHICLE. COMPRESS THE COIL SPRING WITH THE PROPER SPRING COMPRESSOR TOOL. REMOVE CENTER RETAINING NUT HOLDING THE UPPER MOUNTING INSULATOR. SLOWLY DECOMPRESS THE COIL SPRING.REMOVE THE MOUNTING INSULATOR AND THE COIL SPRING. REMOVE THE COIL SPRING. TO INSTALL COMPRESSED COIL SPRING ONTO THE STRUT ASSEMBLY. BE SURE TO THE END OF THE SPRING IS IN THE NOTCH ON THE LOWER SEAT.INSTALL UPPER MOUNTING INSULATOR. YOU WILL NEW LOCKNUT FOR THE CENTER RETAINING NUT OF THE COIL SPRING. CENTER RETAINING NUT AND TORQUE THE NUT TO 13 TO 17 FT LBS. SLOWLY REMOVE THE SPRING COMPRESSOR TOOL. INSTALL SHOCK ABSORBER ASSEMBLY IN THE VECHICLE AND INSTALL THE MOUNTING FASTERNERS AND TORQUE LOWER MOUNTING BOLTS TO 72 - 87 FT LBS. INSTALL WHEEL.LOWER VECHICLE. WARNING COIL SPRING UNDER EXTREME LOADS WHEN COMPRESSED.BESURE TO PROPERLY ALIGN THE SPRING WITH THE COMPRESSING TOOL TO PREVENT PERSONAL INJURY FROM THE SPRING RELEASING UNEXPECTEDLY.

REMOVAL & INSTALLATION 1983-86 Models

1. On 4-door sedan, remove the package tray and vent duct.
2. On hatchback, remove the speaker grilles.
3. Disconnect the brake line from the wheel cylinder. CAUTION
   Brake fluid contains polyglycol ethers and polyglycols. Avoid contact with the eyes and wash your hands thoroughly after handling brake fluid. If you do get brake fluid in your eyes, flush your eyes with clean, running water for 15 minutes. If eye irritation persists, or if you have taken brake fluid internally, IMMEDIATELY seek medical assistance.
   
4. Remove the brake line from the brake hose.
5. Disconnect the brake hose from its bracket on the shock absorber.
6. Loosen, but do not remove, the nut holding the suspension support to the shock absorber.
7. Unbolt the shock absorber from the rear arm.
8. Unbolt and extract the shock absorber from the body. To install:
   
9. Attach the shock absorber to the body and tighten the 3 nuts to 17 ft. lbs. (25 Nm).
10. Engage the shock absorber bracket with the carrier and install the hardware. Tighten the nuts to 119 ft. lbs. (162 Nm).
11. Tighten the center suspension support nut to 36 ft. lbs. (49 Nm). There is a bearing located under the suspension support dust cover. Remove the dust cover and pack the bearing with multi-purpose grease. Once packed, reinstall the dust cover.
12. The remainder of installation is the reverse of removal. Tighten each component to specifications.
13. Have the rear wheel alignment checked.

1987-88 Models

1. On wagon models, remove the toneau cover holder.
2. Loosen the rear wheel lug nuts, raise the rear of the vehicle and support it safely. Block the front wheels.
3. Remove the rear wheels.
4. Using the proper size flare nut wrench, disconnect the backing plate hardware.
5. With a flare nut and a back-up wrench, disconnect the brake tube from the brake hose and use a small plastic container to collect the brake fluid.
6. Undo the clip and disconnect the brake hose from the shock absorber. NOTE: Before the axle bolts are removed, the axle carrier must be supported with a jack.
   
7. Support the axle carrier with a floor jack and remove the mounting bolts and nut. Disconnect the axle carrier from the shock absorber.
8. Support the shock absorber firmly by hand and remove the three mounting nuts. Remove the shock absorber from the body. To install:
   
9. Position the shock absorber onto the body and support it firmly by hand. Install the three nuts and tighten them to 23 ft. lbs. (31 Nm).
10. Attach the axle carrier to the shock absorber with the nuts and bolts. Tighten the nuts to 166 ft. lbs. (226 Nm).
11. The remainder of installation is the reverse of removal. Tighten each component to specifications.
12. Bleed the brake lines as described in Section 9.
13. Have the rear wheel alignment checked.

1989-96 Models

1. On the 4-door sedan, remove the package tray and vent duct. On 1992-96 models, it will be necessary to remove the rear seat back.
2. On the hatchback, remove the speaker grilles.
3. On the wagon models remove the tonnue cover and rear seat back.
4. Loosen the rear wheel lug nuts, raise the rear of the vehicle and support it safely. Block the front wheels.
5. Remove the rear wheels.
6. Disconnect the brake hose and the ABS speed sensor from the shock absorber.
7. If equipped with ABS, disconnect the Load Sensing Proportioning Valve (LSPV) spring from the lower arm.
8. Disconnect the stabilizer bar link from the shock absorber.
9. Support the rear axle and loosen the 2 bolts from the lower end of the shock absorber.
10. From inside the vehicle, unbolt the upper support. Fig. 1: A rubber cap covers the rear shock absorber tower nut
    Fig. 2: The upper shock absorber tower is retained by the center nut (circled) and three surrounding nuts
    
    
11. Lower the rear axle carrier and remove the two bolts.
12. Remove the shock absorber from the vehicle. To install:
    
13. Install the shock absorber assembly onto the vehicle.
14. Connect the brake hose and the ABS speed sensor to the shock absorber.
15. Connect the stabilizer bar link to the shock absorber.
16. Install the wheel and tire assembly.
17. During installation, please observe the following tighten specifications:
    1. Tighten the shock absorber-to-body bolts to 29 ft. lbs. (39 Nm).
    2. Tighten the shock absorber-to-axle carrier nuts to 166 ft. lbs. (226 Nm) on 1988-91 vehicles and 188 ft. lbs. (255 Nm) on 1992-96 vehicles.
    3. Tighten the suspension support-to-shock absorber nut to 36 ft. lbs. (49 Nm) on 1988-91 vehicles. On 1992-96 models, tighten the stabilizer bar link to shock absorber retainers to 47 ft. lbs. (64 Nm).
18. Install the rear seat back, vent duct and the package tray as required.

The struts model (MacPherson Strut),are both in front and back of this car.
2000 - 2006 Hyundai Elantra Shock Absorber and Strut Assembly - Rear, Driver Side, KYB GR-2/EXCEL-G, Twin tube, Strut Assembly. $72
2000 - 2006 Hyundai Elantra Shock Absorber and Strut Assembly - Rear, Passenger Side, KYB GR-2/EXCEL-G, Twin tube, Strut Assembly $68
1996 - 2006 Hyundai Elantra Shock Absorber and Strut Assembly - Front, Driver Side, KYB GR-2/EXCEL-G, Twin tube, Strut Assembly $67
1996 - 2006 Hyundai Elantra Shock Absorber and Strut Assembly - Front, Passenger Side, KYB GR-2/EXCEL-G, Twin tube, Strut Assembly $63
Of course this is only the cartridge, you have to transfer the spring, and the top mount of the existing strut. You need a pair of coil spring compressor to do the job. The basic procedure to replace a strut is:
Remove the top nuts that secure the strut assembly to the frame mount inside the engine compartment. Do not remove the center nut.
Mark the lower strut mounting location on the hub for installation reference. The alignment will be close if the new strut is installed in the same position.
Remove the bolts that hold the strut assembly to the hub.
Remove the strut assembly from the vehicle.
Mark the top mount for installation reference. Using a side mounted spring compressor, compress the spring and remove the top nut and retaining washer. Pull strut assembly out from the bottom of spring.
Slide the new strut assembly up into the spring and reposition the top mount onto the spring in the same position that it was removed. Install the retaining washer and top nut.
Tighten the top nut to the manufacturer's specifications and carefully remove the spring compressor.
Place the strut assembly back into the vehicle.

The strut would not extend. Strut are made to go inside of the coil spring, and held in the compressed position.
If the strut is sticking past the coil spring something has gone wrong. You'll need to compress spring and fix it.

Installation

1. CAUTION: The bearing and seal assembly is press-fit onto the upper mount. The mount washers must be installed in the correct orientation.
   NOTE: Be sure to check the spring insulator for damage before assembly. If the outer metal splash shield is bent or damaged, it must be carefully bent back, so it does not touch the locator on the bearing and seal assembly.
   
   NOTE: New gas struts are shipped with the rod in a locked down position. To extend the new strut, remove the external tie down and/or rotate the upper end of the rod, counterclockwise.
   
   Install the coil spring insulator, coil spring, and upper mounting parts to the new strut assembly. Make sure the spring is correctly seated in both spring seats.
   

Item Part Number Description 1 — Dust Boot (Part of 18045) 2 N803826-S100 Front Shock Absorber Mounting Bracket Nut
(3 Req'd) 3 N803952-S100 Washer 4 N808625-S100 Front Shock Absorber Nut 5 18183 Front Shock Absorber Mounting Bracket 6 3B455 Front Suspension Bearing and Seal 7 5310 Front Coil Spring 8 5414 Front Spring Insulator 9 18045 Front Shock Absorber A — Tighten to 35-40 Nm
(25-30 Lb-Ft) B — Tighten to 55-63 Nm
(40-46 Lb-Ft)

1. CAUTION: Do not over tighten the lock nut; damage to the lock nut or piston rod could result.
   Install new rod lock nut and tighten it to 55-63 Nm (40-46 lb-ft).
   

1. Remove the spring compressing tool.
   

1. Position the strut assembly into the vehicle. Install the three mount-to-strut tower nuts. Tighten the nuts to 35-40 Nm (25-30 lb-ft).
   

1. Partially raise the hoist.
   

1. Position the strut assembly to the steering knuckle. Install the pinch bolt and tighten it to 115-132 Nm (85-97 lb-ft).
   

1. Position the stabilizer link to the strut assembly and install the retaining nut. Tighten the nut to 90-100 Nm (66-74 lb-ft).
   

1. Install the front wheel and tire assembly.
   

1. Lower the vehicle.
   

1.Turn ignition to OFF position to place steering column in unlocked position.
2.Remove hub nut
3.Loosen, but do not remove, three top mount-to-shock tower
4.Raise vehicle on a hoist.
5.Remove tire and wheel assembly
6.Move brake caliper and wire out of the way
7.Remove brake rotor
8.Remove cotter pin from tie rod end stud and remove slotted nut.
Discard cotter pin and nut
9.remove tie rod from knuckle
10.Remove stabilizer bar link nut, and remove link from strut
11.Remove and discard lower arm-to-steering knuckle pinch bolt and nut. (A drift punch may be used to remove bolt.) Using a screwdriver, slightly spread knuckle-to-lower arm pinch joint and remove lower arm from steering knuckle.
12.Press halfshaft from hub
13.Remove shock absorber strut-to-steering knuckle pinch bolt. Using a large screwdriver, slightly spread knuckle-to-strut pinch joint, if required, for removal
14.Remove steering knuckle and hub assembly from shock absorber strut
15.Remove three top mount-to-shock tower nuts and remove strut and spring assembly from vehicle
16.Compress spring with Spring Compressor
17.Place 10mm box-end wrench on top of shock strut shaft and hold while removing top shaft retaining nut with a 21mm 6-point crow foot wrench and ratchet
18.Loosen Spring Compressor , then remove top mount bracket assembly, bearing plate assembly and spring.
INSTALLATION
1.Place a 10mm box-end wrench on top of shock strut shaft and hold while tightening top shaft retaining nut with a 21mm 6-point crow foot wrench and ratchet to 53-72 Nm (40-53 lb-ft).
2.Install strut and spring assembly and three top mount-to-shock tower nuts
3.Install steering knuckle and hub assembly to shock absorber strut.
4.Install a new shock absorber strut-to-steering knuckle pinch bolt. Tighten to 98-132 Nm (73-97 lb-ft).
5.Install halfshaft into hub
6.Install lower arm to steering knuckle ensuring that ball stud groove is properly positioned and boot seal is not damaged. Install a new pinch bolt and nut. Tighten to 53-72 Nm (40-53 lb-ft).
7.Install stabilizer bar link to strut and install a new stabilizer bar link nut. Tighten to 77-103 Nm (57-75 lb-ft).
8.Install tie rod end onto knuckle.
9.Install a new tie rod end slotted nut. Tighten to 31-47Nm (23-34 lb-ft).
10.Install a new slotted nut retaining cotter pin.
11.Install brake rotor
12.Install brake caliper
13.Install tire and wheel assembly
14.Tighten three top mount-to-shock tower nuts to 30-40Nm (23-29 lb-ft).
15.Lower vehicle and tighten hub nut to 230-275 Nm (170-202 lb-ft)
16.Depress brake pedal several times prior to moving vehicle.

Hope this helps. Good luck !