Try taking off the tunnel ram and switch to a dual plane intake. Tunnel ram run really crappy for a street driven vehicle as they are made to make power at over 3000 rpm (not good for daily driver).And, since you said you are running dual Holley 450 cfm. On backfire can destroy you power valves, and that may be your problem. Try an Edelbrock. As for the backfiring problem, make sure the distributor is in right, maybe when the engine was assembled the distributor was off 180 degrees. Also, check the firing order. Make sure all of your head and intake bolts are properly tourqed and the are no leaks under any of your gaskets.