If your not re-using the mufflers get a series of flathead screwdrivers and stick the biggest one you can fit between the pipe and flange and bend away. Progress to a bigger and bigger screwdriver and keep trying to turn and wiggle it, and it will come out
I just did this (I had flowmasters before and the shop welded the snot out of the inlet and esp the outlet). I just used the cutoff wheel and cut right at the outlet flange on the muffler (to preserve as much tail pipe as I could). You might find that you have to do some boogering on the tail pipe where it exits the muffler to make that connection (the pipe takes a quick bend upward to clear the axle, and you might end up trying to put a bent pipe into a straight muffler exit). The shop that had done the great welding had cut the oem system off and left me no straight meat at all in the end of the tailpipe (the old muff's were 2.5" and so there was room for them to angle the OEM 2.25" pipe into the outlet. )
I only post this so you can size it up before cutting (so you know what you might run into).