The ole girl ('92 Lumina) is running pretty rough, it blew the manifold bushing out so I figured that the cat. converter is clogged. My question is, what is the best way to change, remove, or gut the converter? It looks like it is factory welded. Do I have to cut it off? Is there some type of sleeve that I can use to put another one back on?

It is easiest and cheapest to go to a discount exhaust place, and have a new universal cat welded in. Whenever you start fooling with cutting and using reducers, it is more trouble than the small amount of money you save. And I am speaking as one who has access to a shop and a rack, it's still a pain to try to "sleeve in" an exhaust part that was originally welded.