The shaft will not go all the way into the flywheel

Not really.
The 3100 is a Chevy 2.8 and the 3800 is a Buick engine.
Most everything is different including exhaust manifolds, engine brackets, and wiring.

Usually the bendex motor is worn out. That motor pushes the shaft into the flywheel as the second main motor "the starter" spins the engine. I had this problem in my 1997 Saturn SW2 and replaced the starter 15 years ago and it never happened again. Often you can replace just the bendex but it can be easier just to get a rebuilt starter at the auto store like I did and replace the whole thing. Warning, it is possible but very unlikely that the flywheel is damage.

you would need to change a lot exhaust the computer the pumps the trans might even be different motor mounts the sub frame could be different as well so unless you have the whole car to swap stuff over its not prattical and if you have a state that test emissions you might never get it to pass

Depends on what sub-model you have. If you have an SE its a 3100 3.1L Engine. If its a GT its a 3800 3.8L Engine. If its a GTP its a 3800 3.8L but with a supercharger.

There are two different size flywheels used on Chevy engines.The straight across bolt pattern is for the small diameter flywheel the staggered pattern for the large flywheel.The block is drilled for both starter patterns.The F- bodies in 1986 came with a small flywheel-so you should need a starter with the straight across pattern.The depth doesn't change ,just the distance away from the crank.

I usually use an impact wrench,but it seems you dont have one.Try holding the engine with a screwdriver at the fly wheel!May have to have some help but I have done it before!

Why do you want to do this, its a big job for the unskilled.

Remove drive shaft / gearbox / clutch assembly (If manual) / Flywheel.

i would recomend double check but am almost positive because is been the same torque for many years for this engine and is 33 ft.lbs plus 90 degrees and the sequence

6 2 3 7
5 1 4 8
front of the engine lefth head(front of the car)

5 1 4 8
6 2 3 7 rigth one (firewall)

REMOVAL:

1. Lift clutch pedal to uppermost position to disengage clutch and brake pedal pivot shaft.
2. Push clutch and brake pedal pivot shaft forward and unhook clutch release lever cable from clutch and brake pedal pivot shaft and allow it to slowly swing rearward.
3. Raise and support vehicle.
4. Remove clutch release lever dust shield.
5. Disconnect clutch release lever cable from clutch release shaft.
6. Remove retaining clip, then clutch release lever cable from flywheel housing.
7. Remove starter motor from flywheel housing, then engine rear plate to front lower flywheel housing bolts.
8. Remove transmission.
9. Remove flywheel housing back just far enough to clear clutch pressure plate, then remove housing.
10. Remove clutch release shaft from flywheel housing by pulling it through window in flywheel housing until retainer spring disengages from pivot.
11. Remove clutch release hub and bearing from clutch release shaft.
12. Loosen six clutch pressure plate bolts evenly to release spring tension gradually and avoid distorting clutch pressure plate. If same clutch pressure plate is to be installed, mark plate and flywheel so pressure plate can be installed in its original position.
13. Remove clutch pressure plate and clutch disc from flywheel.

INSTALLATION:

1. Position clutch disc and pressure plate assembly on flywheel, noting following:
   1. Three flywheel housing to block dowels on flywheel must be properly aligned with clutch pressure plate.
   2. Bent, damaged or missing flywheel housing to block dowels must be replaced.
   3. tart clutch pressure plate bolts but do not tighten.
   4. Avoid touching clutch disc face, dropping parts or contaminating parts with oil or grease.
   5. Align clutch disc using suitable alignment tool inserted in pilot bearing.
   6. To avoid clutch pressure plate distortion, alternately tighten bolts a few turns at a time, until they are all tight, then tighten to specifications.
   
   
2. Install transmission to flywheel housing.
3. Install engine rear plate to flywheel front lower housing bolts, then connect clutch release cable to flywheel housing and connect retaining clip.
4. Connect clutch release lever cable to clutch release shaft, then install clutch release lever dust shield.
5. Install starter motor.
6. Lower vehicle, then install clutch release lever cable as follows:
   1. Lift clutch pedal to disengage clutch and brake pedal pivot shaft.
   2. Push clutch and brake pedal pivot shaft forward and hook end of clutch release lever cable over rear of clutch and brake pedal pivot shaft.
   
   
7. Cycle clutch pedal several times to adjust clutch release lever cable

no your on the rite track worst case you will have a engine light on no big deal