Backfiring

I have a 1971 GTO with a Holley 800cfm and a 400 motor. I had a Q-Jet carb but switched it with the holley. At low rpm's (1000) when i tromp on it I get a backfire, but at high rpm's (over 1500) it runs great. The carb came off of another car and you can tell they had the same problem because the choke plate is all dented. Just wondering what I have to do to fix this problem.