backfires

As stated in the title, my cobra is sputtering and backfiring below 3 to 3500 rpms then smooths out, it seems to me like its missing fairly bad. Its hard to tell during idle since it has stage 3 cams installed and i'm fairly new to this car so I cant say for sure. It's also worth mentioning...

I've got a '95 cobra 5.0. I recently finished a rebuild on the motor. Stock bottom end, mild cam, valve job and slight porting on the gt40 heads, port matched and polished bottom intake, 75mm tb, 76mm mass air, bbk cai, and longtube headers, 180 t-stat. The problem I'm having is I have no power...