Ted,
I hope that you had a restful night. I hear your frustration. As I thought about it last night, this is what I came up with regarding things to consider based upon these premises: 1. When you had the Webers on previously the backfiring was not a problem, or else you would have mentioned it. 2. The backfiring was not a problem with the Holley.
What have you done to the engine in order to put the Webers back on, you had to remove the Holley and intake manifold and the distributor and re-adjust the fuel pressure. I doubt that the issue is with the intake manifold having a leak, as if it were sucking air it would also be sucking
oil from the gallery and you would notice that on the plugs.
Did you thoroughly clean and inspect all of the jets, air correctors and holders? It does not take more than a small speck to clog or interfere with their function. Drain your fuel bowls and see if anything comes out.
Another possible culprit is the distributor. Are you using a bronze gear on the shaft at the cam? Bronze gears wear (that is their primary purpose, to sacrifice themselves instead of the cam gear) and they really do not last that long. Drag racers replace them often as they rebuild their engine often. On my recent build with the road race engine the bronze gear was showing significant wear after only 4 hours of run time. The engine builder stated that it was affecting the timing. It may be possible that it is slightly off on the camshaft. I would pul the valve cover and check to make sure that you are at TDC on #1 and see where the pointer is showing for timing.
Good luck, hopefully someone with more knowledge than I will chime in. Those seem to be the mostly likely culprits, as those were the things that were changed.
Jim