Backfiring through the carbs is more likely caused by an overly lean (not enough fuel) condition. Most likely a inoperative choke. Also it has come to my attention (by letting my girls drive my old cars)that most young owners of "old" cars (has a carb)do not understand the concept of "pump the gas a few times before starting". This isn't their fault, since fuel injected vehicles have always done this for them. Also, if the problem is spark related (and not caused by crossed up plug wires, double check your firing order) it would have to be that the timing is retarted (slow) since the intake valve closes before the piston makes it very far at all from bottom dead center, otherwise it would pump the compression back into the intake manifold,(this can happen with a broken intake valve spring, or bent intake valve, missed any wide open power shifts lately?)
jess