Bolting the flange down while welding won't do ---- if you don't control your welding and heat etc. I've watched plenty of things that were bolted solid spring twisted as ---- after they cooled and were unbolted.

Its an art to weld stainless and not have it warp. I saw a 1/8 stainless sheet that was welded to 3/16 wall 2x3 steel tubing bend the tubing 3 feet in a 15 foot run.