No problem welding the stainless, I have lots of MIG time but this was my first stainless. I had to use mild steel argon/CO2 gas with stainless wire, since that was what was available; it worked, but the welds were ugly.
Preparation grinding is key to getting it welded shut, with beveled edges and CLEAN surfaces to weld upon.

The car was VERY quiet before when clogged, now it sounds just a tiny bit more throaty than stock, not loud at all. There's still a lot of muffler after that cat can.