I did a little reading on mig welding stainless steel. Seems "tri-mix" gases of varying percentages are the most often recommended. The third gas is oxygen. The majority is argon, with more argon than the usual 75% for steel (often 85-90%). Didn't see any recommendations for 100% argon.

Bottom line, it kind of sounds like you'd have to pioneer it for your own application as to which gas to use. And that would run some $ for a small job. I'm confident you could use 308 or 309 wire, and whatever gas you had on hand to tack the exhaust system.

Bitchin57, were those welds black after doing some wire brushing?

The above is just based on a google search, all my stainless welding experience is with tig and 100% argon. You use 100% argon to tig weld carbon steel too. And I will say this, my limited experience with tig welding stainless is that I've had great results and it welds really nice. For me, easier than carbon steel if you're welding on 16 gauge.