If it was a Flowmaster, you should have just left it off. That sucks about the muffler thing, though. Next time just run without mufflers and you'll be fine.

On a sidenote: header gaskets suck. I'm sure one of mine are blown again. The back bolts are such a pain in the ass to get to on a 4th gen fbody. Plus you need to rig your own wrenches to be able to get like 1/8th of a turn on them around the tubes.