Buzz,
The one spade you mentioned is for ground, make sure it is actually grounded to the carb. The second is a "key on" hot side. This should be hooked up, even where you're at, just to make sure the butterfly is wide open.
But, the down side, I don't think this does have anything to do with the backfire.
What side is it restricted to? Both?
My guess's:
Exhaust..you did state new exhaust was about the only change to the motor. Look at all the connections close with a bright light. Look for any soot around header flanges, slip tubes, head to header area.
Lifter has failed to totally pump up yet
Lifter is partially collapsed, sticking somewhat open
(Usually, some good running time at moderate rpm with the engine totally warm will solve both the above problems)
You've simply crossed a plug wire
It's the damned Holley! If you try all things mentioned, and nothing works,... it's the damned Holley!
Edelbrock 750, w/electric choke, simple bolt in mod and will work!
Let the flames begin!
DV