butt welding exhaust system

That does look good but frankly I think some of you are over-thinking this a bit. In my "yut" I've welded a few with oxy / acetylene. Aren't we talking about "non show car" "street car" here? Most of the stuff I've done had already had exhaust through it, no real need for inert fancy back purging.

And so far as that goes, CO2 works pretty well and it's one 'ell of a lot cheaper.