To answer your question.....
Any shop worth their salt will be able to custom fit just about any muffler you choose. Again, at our shop we do it every day.
A good stainless steel muffler will last for quite a while. You might even get lucky and find a shop that will warranty their mufflers.
But, the other guy is right, wherever you weld on a stainless steel system is where you are likely to get rust. Solve that problem by painting the welds. There might be a special paint, if not any good paint will work - use high heat.