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Chub, if you're looking to keep them, try some PB Blaster, spray in the sections, rap a little with a hammer, spray again, rap, spray again and let it sit overnight. It't still stuck, cut. Bill.
I leave the front pipe connected to the engine, disconnect the back hanger and wiggle, rotate, pull and curse the muffler as my feet are propped against the car somewhere. If the system is in good shape I'd hang on to it in case you want a break from the side pipes in a few years. I assume you have the rear panel off since it will be replaced, adds more working room. And, safety first when under the car manhandling parts. Good Luck
If you don't have an acetylene torch to heat it, try laying a 2 X 4 against the muffler and see if you can drive the pipe off at the transmission joint. Leave it connected to the engine. It may work.
Eastwood sells a pipe separator. I've mever used one but it should work. I doubt the penetrating oil will do much good as it is the crimp on the pipe that is hard to separate, not rust.