I thought the muffler was made of a 304 stainless - but am not absolutely sure. What I do know is that I welded my muffler cracks with 308 stainless wire on a MIG welder and it has held so far.
I just got a TIG welder and am going to weld in an Air/Fuel bung into the muffler this week. I'll still use some 308 filler rod for the weld.

Regards
Rodney