1979 Chevy Monte Carlo backfiring

I have a 1979 monte carlo with a new 330 hp gm engine. About a week anfter I got it I was working on a brake problem and a slow neighbor of min dectied to see if he could move my Distributor. Well needless to say he messed it up. It tooke me a few tryes and borrowing a timming gun but I got the timming back. Sence then though it seemed to run fine at first but now it started backfireing again and I checked the timming and its fine but its backfireing both threw the carb and the tailpipe.