[914fan]

Definatly look listen for an intake leak. That would cause a lean idle condition that WILL cause backfire through the carb. Also check that the corect size idle jets are in the carb (i usually play with webers but i'm sure that a holly is simular in function) Also you could have no problem other than you gave it tooooo much fuel when you tried starting. Now that it fired through the carb you have one or more cloged jets. You may also have burnt / melted whatever. Pull the carb and look at it close. You probably do not need to re-rebuild it but look at it. Blow it out. check the float setting. check your fuel pressure. Basic stuff.
But if I were to put money on it I would say intake leak.