Replacing the gasket between the cat and muffler.

I need to replace the gasket at the flange between the cat and the muffler. I would think a pretty easy job. Obviously not. The bots are studs. The bottom two snapped. So I now have get the back end of the studs out which is a pain as it is. But the upper part seems to be a stud in a welded threaded thing. I am beyond confused. Because how could you thread a stud in with a round back. I tried cutting that area off the flange and smoked 2 sawsall blade. I have no idea what to do here. Any ideas?