Have a 76 Dodge D300 with a 440 motor that fires but won't always start.

If no spark check out the wiring in ignition circuit along with your ecm which fires the plugs.

Sounds like timming to me?

how much is the RPM showing in the datatlogger while cranking?

your rotor may be cracked or try the ignition coil wire. it may be loose

1. Ignition Switch: Improperly Connected or Faulty Ignition Switch.
2. Battery: Battery Discharged or Faulty or Coroded, Broken, Shorted or Poorly Connected Battery Cable.
3. Solenoid Switch: Starter Solenoid Switch Faulty.
4. Starter Starter Motor Faulty.
5. Wireset: Worn or Damaged Spark Plug Wires.
6. Coil -Ignition-: Faulty Ignition Coil.
7. Distributor Cap: Loose, Cracked or Worn Distributor Cap.
8. Spark Plug: Incorrectly Gapped or Fouled Spark Plug(s).
9. Distributor Cap and Rotor Kit: Loose, Cracked or Worn Distributor Cap.
10. Carburetor: Worn, Faulty or Damaged Carburetor.
11. Fuel Pump Faulty Fuel Pump.

sounds like it could be your crank sencer or being a 99 does it have a distributor or a coil over plug ignistion system called a DIS [distributorless ignistion system] , if it has a distributor it could be cap and rotor that you need. best bet is to get a diognostic systems check but if no codes detected then you need to check all in line fuseable links for a dead short,a fried brain box ,or a bad pron in your ecm [electronic control module ]

Firing order okay? the MAF can do this to you (Mass air flow) a bad connection at the distributor can do this. The distributor connection can do this if the 180 signal is missing going to the ECM. So wiggle test that connection and inspect that male to female connection. The MAF (mass air flow) in this particular year it was called a volume air flow sensor so if this signal is missing signal to the ECM it stumbles the vehicle so wiggle test that connection also check fuseable links and ground for this mass air flow.Rule of thumb a backfire always occur when timing is off giving a lean mixture. and a miss under load is a meaning for not enough fuel.

the wires to the plugs should not keep the motor from cranking over. If the battery is fully charged the ignition switch should engage the starter. If you think the firing order is wrong you need to determine 1 where is top dead center (the position the motor is in when #1 cylinder is at the top of the compression stroke). 2 what direction does the rotor turn 3 what is the firing order One way to make it easy to rotate the motor is to remove the spark plugs. no compression means no resistance to the starter so the motor should turn over with the key in the start position. If it won't crank over with the plugs out then something is wrong with the starter, battery, starter relay, or ignition switch. I always start with the basics. the battery should have at least 12.5 volts. if it does check the battery terminals. even if the starter is weak you should hear a click when the key is turned to the start position. Once you get the starter to turn over the motor you can tell when the number one cylinder is at top dead center by putting a finger over the spark hole as you crank over. You will feel the compression as the cylinder comes up on the compression stroke. I have used a large socket to slowly turn the motor over and when you are near top dead center you should be able to see the degrees of advance on the timing mark. Set the motor at top dead center then see where the rotor lines up with the wires on the distributor cap. that is where number one cylinder should fire. You then follow around in the direction that the rotor turns and the wires will corispond with the firing order. I always make a mark on the number one spot on the cap with a felt pen. That way you have a fixed reference point. Take care to verify that all the wires are to the proper plugs before trying to start. Take extra care at this point and make sure your firing order and rotor rotation are correct. I would triple check this. The firing order and rotor direction are usually somewhere on the autozone repair web site

you said you repl dist cap that means you don't have a crank sensor you have electronic ing module. remove your elec ing module have it tested most auto parts stores could test your module, or check your coil at the neg terminal. should have power. if not you lost your primary volt from your batt.your secondary power is coming from your electronic ing module from the module to the pos terminal on the coil, when the eng cranks,from the coil to the dist cap , from the dist cap center to the rotor then out to the plugs,this all happen pervided the timing belt or timing chain is ok. the cumbustion chainber needs 3 things , thats fuel, air, and spark.

Are plugs and wires good? Did you check distributor? Firing order correct-timing set right? Maybe some of the new parts are faulty? Are grounds all secure and are fuses good?