Help please: 454, Holley 750 on Performer, points distributor. Everything ran perfect, installed new fuel pump and intake manifold (didn't remove valve covers). After assembly, it starts and idles fine, runs fine (maybe a slight stumble) under light or moderate acceleration, but stumbles and backfires when stomping on it or hitting it quickly in neutral.
So far: replaced fuel filter, verified TDC and firing order, played with timing (still a problem anywhere from 8 - 18*), replaced plugs/cap/rotor, swapped on another carb (brand new from perfectly running engine). Plug wires are routed properly with looms, they are newer and in good shape. Manifold vacuum is a steady 16 at idle, sprayed around intake and carb to check for leaks. Problem still exists and frustrations are high!
Could it be points and condensor, even though they were fine before the swap? Still a possibility of vacuum leak, but what else to look for to verify?
Any and all suggestions are welcome and greatly appreciated. Thanks.