Charlie,
I think that your path of diagnosis is as Bolter2 stated, and you're already on the way: Fuel delivery, timing, vacuum advance, vacuum leak, exhaust restriction.
Plugs that are over-carboned would indicate a rich condition, though I'm not certain that would lead to backfiring? If they're oily black, well, that's oil in the combustion chamber. Fouled plugs will certainly cause bad acceleration, but again, not certain of the back-firing. "just a bit" on the black side may mean usual wear on the plugs, with a slight rich condition, but isn't related to this problem. On the other hand, a lean condition will give the poor acceleration you describe, and could cause backfiring.
Check for that accelerator pump squirt, and whether the distributor rotates as stated (this is easier with an assistant in the driver's seat). Report back on your findings. Also, describe the backfiring in more detail. Is it "popping" out the tailpipe, a bunch of rapid pops, or one big whammo? Or is it clearly back through the carb?
-Michael
