Help On Removing Axle-backs

I've got a 2006 GT that has Flowmaster axle-backs and I'm guessing the previous owner put those on at least 5 years ago. I picked up a set of Roush axle-backs and tried to swap them in yesterday, started on the passenger side and had no problems with the four hanger bolts, loosened the pipe clamp, but for the life of me I couldn't wrestle the muffler loose. It looks like the winter salt here in the Midwest has corroded the pipes together.

Normally I would just hack and butcher the old muffler to get it separated from the pipe, but I want to keep the Flomasters in decent shape so I can sell them. Any advice on how to break it loose w/o doing any damage?

I have a propane torch (not sure how effective that would be?), but I'm a bit nervous about using a torch with all the plastic fuel lines in the vicinity. I could beat on it with a rubber mallet, but is there any risk of damaging anything farther up the line such as the cats or manifolds? I guess I could spray liquid wrench in there the night before and then try hammering a thin flathead screwdriver in-between the two pipes to try to expand the flowmaster sleeve?

I finally realized why I was having such a tough time getting them off. The previous owner that put the Flowmasters on used a clamp that was two heavy guage wire loops vs the wide strap kind and he way over-torqued the clamp and seriously crimped the pipe. I was able to get the passenger side off by repeatedly heating it, but I couldn't get the driver side to budge after heating the hell out of it (and the gas filler pipe inches away really made things fun). I had to use a dremel on the driver side to make a longitudinal cut to expand the end of the muffler pipe to get it to separate.