Are you *certain* you got the wires right? Possible and pretty easy to hook them up starting in the correct place, but in the wrong direction around the cap...

I bought an old Cadillac once that had no cap or wires... I recearched various GM motors for firing order, and all were the same.. so I figured the distributor rotor turned the same direction.

I was wrong. On that particular Cadillac motor, the distributor turned opposite of about all other GM V8s.. and the backfires sounded similar.

Double check wires, and in the future, always do the wires one at a time.

Only other possibility I can think of is you got wrong parts, but since you've been busy putting old ones back in and with no positive results, I have to lean to incorrect wire placement.

I'll be curious to hear what fixes it...

--
-Matt I ♥ my ♂