Scotty,

Your timing advances from two souces, one is vacuum, the other is centrifugal. The centrifugal is controlled by weights inside the distributer that move in relation to engine speed. So even with the vacuum removed you will still see timing changes from the centrifugal when you rev the motor.

As for tuning, it would help to know which 750 Holley you installed and whether your backfires are coming back through the carb or out the exhaust. If it is from the carb, it sounds like you may need a stronger accelerator pump shot. There should be some different holes in the linkage you can play around with to vary the shot from the pump.

good luck

Pat