Back fire problems

I've got a 73 Duster, 383 40 over, mild street cam
750 holley,Power valve is good,Plenty of Vaccum,
Everytime I get on it , it backfires
I just reset timinig for 28 to 36, seems to solve most of the backfires but still has a small one, How much is too much timing, Do I need more timing or should I be looking for other problems
Any help is good help, Thanks, Greg:banghead::banghead::banghead::tongue10::tard::clock: