Backfire

[MustangMatt69]

I have a 1969 Cougar XR-7 with a 351 Cleveland that is far from stock. It has compression in the 11's with 4 barrel quench heads and a big hydraulic roller camshaft. It also has all of the MSD ignition upgrades in it such as: The 6-al ignition pro billet distibutor 15 degrees timing control, a 750cfm carb with edelbrock intake. When ever I am cruising around it back fires. It usually always backfires when I first hit the gas or get off the gas. I know that it is an exhaust backfire and not intake but what can I do to fix it. Right now I am running premium fuel with no octane booster. Should I add some octane booster and lean out the carb mixture a little bit. The 750 cfm I put on about a month ago, but I havent played around with the mixture screws, do you think that the mixture screws would be a good place to start? The old carb did not backfire nearly as much, maybe once in a blue moon.
Thanks alot
MustangMatt69