I am making my headers out of Mild steel. Stainless is a pain in the butt. You either have to back purge the tubes with argon when you weld them or apply flux to the inside of the tubes before you weld. Plus it is harder on all your tools etc. I guess if you work with it a lot, it is not so bad.

These are going to be 1-7/8 X 2 X 2-1/8 with merged collectors.