Holley 750 backfire.

I got 4.10 gears out back. Now can I do what you suggested to my timing have it all in at 2100 with the stock Mopar distributor? I honestly don't look the way my intake sits on the heads either so I replaced the valley pan gasket today (metal one). Motor had the heads already on them already So there is a chance they could be milled. The guy I got it of said it was out of a 69 charger that would run 10.5 1/4 miles on this motor it's probaly BS but who knows what kind of head work was done or true compression of the engine. I do know that it was balanced and blueprinted though. But anyway if they are shaved I might be getting a leak from it, I did the spray bottle water test but got no change in rpm but maybe it's leaking from the underside of the intake?