Sounds like this is going to require getting back to the basics. You're going to have to take vacuum readings thru the rpm range & install the spark testers to test the ignition.

Where are you sourcing your propane? What type of lp tank are you using? What type of shut off solenoid does your system have?

If you've got a model J impco converter with the correct spring & a new electronic distributor, then it's time to look outside the box. Where did you source the dist cap, rotor, wires & spark plugs. I personally have had no luck with tvh parts in the tune up/electronic realm.

Are you sure the ignition coil is receiving a full 12 volts thru the entire rpm range? All these suggestions are all off the wall but they do rear their head once in a blue moon