What's great about acetylene is that it heats up the nut faster than the inconel. Shouldn't ever run into that problem.Are you talking about the downpipe/cat to exhaust pipe connection, or the head-to-manifold connection?
The downpipe connection uses Inconel bolts and copper plated steel nuts. Do everything you can to avoid snapping the Inconel bolts. New bolts are $20+ each and you'll be spending that much for the drill bits to drill out the snapped one.
Inconel is OK to heat up to 1300F, somewhere between "blood red" and "dark cherry". That's the best way to remove a rusted on nut.
An exhaust shop that uses a "heat wrench" to make them glow orange will damage their grain structure. Not a problem for them, you are the one that will pay in the future. Similarly knocking out the bolt with a air hammer risks breaking the catalyst and oxygen sensors -- a future problem for you.
The only advice for the heat-to-manifold nuts is to let them soak for a day, and expect to replace a few studs. PB Blaster in a spray can is my favorite. It would be Aerokroil, but that stuff climbs up out of the can at night and dissolves the neighbor's robo-cats.
Always replace the copper-coated nuts. Using them strips away the copper coating, leaving exposed steel. I can get away with sometimes reusing them in dry parts of California, but re-used nuts will rust in a few weeks elsewhere.