Thanks for the responces. To answer a few questions...

Heads have new valves, springs etc. assembled at Edlebrock. I was pretty meticulous about setting the lash being sure not to crush the lifters. I will keep that in the back of my mind though.

The crane distributor does not have springs... it has 9 pre-set curves that are selectable with a turn of a switch. I have been playing with that trying to find the "right" curve for my engine, but that doesn't seem to affect the backfire issue.

I just had the Jacobs ignition box tested at the factory and I havn't changed the rev limiter. I should be factory set @ 10,000 rpm. A rev limiter seems a little pointless for a street vehicle with an automatic trans.

I agree that I do need larger squirters for the accelerator pump... I do get a small single pop right when I mash the gas, but that is a different issue. This backfiring occurs well beyond the use of the accelerator pump after I've had the pedal mashed to the floor for a few seconds and the engine has already been accelerating. By this time the engine has to have used up the pump shot.

The backfire comes ou the exhaust. When I lift the throtlle a bit and keep it in 2nd gear, the engine will still continue to accelerate so I'm pretty sure it is not a rev limit issue.