Have you checked the plugs since you installed them? I have had problems in the past with a plug that went bad almost right out of the box, it ran fine on idle or slow excelaration but missed on hard accelaration.
If you have a set of torches at home, turn on JUST THE OXYGEN and as the car is idleing run the torch head with the leaking oxygen around the intake and carb gasket area. If the engine picks up RPM, then you have a leak in the gasket and its sucking in the oxygen wich causes the higher RPM.
In a quick check, usually, but not always, backfiring threw the carb is usually a electical problem and backfiring threw the exhaust is a carb problem, unless you have a intake valve not closing properly, then it will blow the compression back up the intake.
One place that alot of people forget about is the bushings for the throttle plate shafts. If they are really worn it will suck air in there also.
A month ago i had a hard time trying to keep my engine ideling between rounds, only to find out that one of the adjuster screws for the idle side was missing. What were the chances of that happening.