I have been a Dodge Auto
tech for 15 years and from what I gather your backfire is due to your distributor housing that holds the pick up coil need to be set in phase with the crankshaft to provide the proper timing from the computer to fire the plugs and injectors. I would advise taking it back to whoever rebuilt the motor to check the distributor sync. Also, there was a recall on distributor drive gears prematurely wearing out if there is no excessive play on the distributor drive shaft to cam then more than likely you are ok there! BOTTOM LINE IS the shop that did the rebuild should stand behind their work and correct the problem, or point you in the right direction.