I've thought some about making a making all-thread type jacking screws as a non destructive method of pushing crimped pipes apart for muffler removal.
Get a band type pipe clamp.
Weld 2 or 3 sturdy stubby ears onto it.
Ears will be flatstock or angle iron, about 3/8 web thickness, maybe 1 inch wide, cut about 1 1/2 long with one 3/8 hole drilled in each ear to carry 3/8 all thread jack screw.
Weld ears onto pipe clamp crossways at 120 or 180 degree spacing.
Cut all thread long enough to reach from ear to end of muffler plus enough additional length to push muffler stub pipe off tailpipe.
Install pipe clamp onto tailpipe, where muffler pipe stub ends.
Using 3/8 nut on all-thread and jacking against welded ear you can use 9/16 end wrench to easily equally turn jacking nuts to push all thread against end of muffler body to seperate muffler stub from tailpipe.
No heating, beating, sawing, bruised whatever, no sweat, shouldn't take much wrench force.
No damage to muffler, can easily be re-used.
If you're seperating two straight pipes or cone shapped CAT, then use 2nd pipe clamp & ears setup clamped onto pipe stub to jack against.
Once muffler is off you can use body tools to easily peen out crimped area for easy work should you decide to reinstall muffler at some later time.
Seems to me like this custom tool should work and is easily made.