[Tommy]

I always start with the basics: compression, fuel and air in the right mix, and ignition spark at the right time. As you had the heads off, all possible causes are on the table. If it were mine, I'd pull the plugs and do a compression test to confirm the valves are set correctly. Next, I'd confirm the position of the distributor/rotor when the engine is at #1 TDC. Then run the plug wires starting from #1. As the fuel mix is less likely to cause the backfire, I'd button it up and try again. If the problems persisted, I'd look to test, replace or borrow a different distributor to see if that is the problem. Good luck.