Usually the bolts just twist off and break when they're that rusted. If you're reusing the flange, you'll need to drill & tap new holes or use bolts. That said, I hate doing muffler work on the ground on my back & usually let the local muffler shop install. Simple things he does for not much $$ and I always pay him cash.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.