I have a 76 firebird with a 455 rocket and it backfires heavily and i believe it is fron the carb but there is no soot or anything at all, when i put the gas peddle down.it loses power and.backfires

I have a 1985 gmc swb with a 350. I have done some changing around tho. I have used a 1987 chevy block and put a inline fuel pump along with a regulated for the fuel system and ran a carb on it. The problem I am having is it wants to backfire threw the tail pipe. I have tried to set my timing...