Backfire through carb at WOT?

I just recently rebuilt my 350 sbc engine, the engine will start fine and idle fine, but when i seem to quickly give it full throttle it will backfire through the carb, the carb is an edelbrock performer 600cfm electric choke, has performer plus cam, rebuilt machined 882 heads, new carb, distributor cap and rotor. ive read a lot of forums and a few lead to bad distributor, i am using a used stock distributor so that could be my problem, i know my plug wires are right, im thinking it could also be valves not adjusted right, my spark plugs are a little black and sooty looking, i bought them brand new, i have vacuum advace and its connected to the passenger side of the carb, thats how it was connected b4 i pulled and rebuilt engine, but the engine will idle fine and rev fine if i rev slowly and not quickly, it only backfires when i flick the throttle to full throttle, any help would be greatly appreciated, thx in advance