No need to purge the tube, this isn't a seawater line or high pressure air system. Around 65-80 amps, tack on four sides, then link the tacks. 309 filler would be fine, and it will discolor. You can try something like sta-brite, or just wire brush it while hot. If you want it really shiny, scrub it with a scotch-brite pad after.
I TIG decorative stainless all day at work, used to do heavy structural stuff in stick, MIG, dual-shield, and TIG.