331 backfires slamming 2nd

Hey guys i recently purchased a 95 gt 5 spd with a new 331 stroker dss pro bullet i just noticed here lately it wants to backfire when i slam the gas after shifting to second rather fast I wondered if it could be a back pressure issue? it has shorty headers, h pipe, old flow masters. When i got it had turndowns about 7 inches out of the muffler I had turndowns took of and the original catback put back on it feels like it's not as fast maybe just cause it's not as loud as it was . if it is the exhaust what will be a good way to go turndowns where nice but u couldnt here nothing inside the car LOL oh yeah when it backfired it cut out and sputtered for a sec and then went right back to normal.