Holley 750 backfire.

Well got it all in today except the timimg issue and it does the same thing :(. I'm ruling out the carb as the problem. Can it possibly be a problem with the ecu box being from a small block vehicle? Plug gap maybe? There tapped at 35. And there autolite 85 plugs. Even through a edelbrock dual plane on it and same thing, I hope it's not something inside the motor? It will rev up in park good but it alway happens when I punch the throttle car spits throughout the carb. Good it be a fuel issue? I am running 89 octane. And the engine has the trw six pack pistons. I'm going crazy!