Use BF torch. Preferably oxyacetylene, heat pipe red hot and they come apart.

BTW, I generally skip the resonator or rear muffler and just put a straight piece of 2.5 inch pipe to the back with a chrome tip or twin slant pipe tipe. Use a reducer to get from 2.5 to whatever the stock pipe is, what, 2 1/4.

The first muffler behind my cat appears to be part of a welded pipe/muffler assembly so I just replace the entire system from the cat back sans the rear resonator.