Heat for any exhaust system removal is key.

I'm in the process of replacing head seal's on one of my Vanagon's, and
prior to even trying to remove the stud nuts / bolts I heated them with
an oxy acetylene torch and the process was simple, clean and instant.

Don't even attempt using a propane torch, it's a waste of time and
energy.
The heat that propane puts out is not focused and not hot enough.

You need to get the flange, nut / bolt white hot prior to removal, and
the Home Depot propane tool just won't do the job.

( 9 times out of ten the fastener will be rusted solid within the pipe
flange )

Invest in a small handyman torch set up.
It'll make life easier in the long run for exhaust work.