Feb. 12th update:
I got the exhaust manifold done! WOO HOO!

Thanks for a new carbide honing bit from Performance Tool, as well as a steel-cutting hole-saw, and a new miter saw, the manifold got put together in a few evenings. Made from the stock 3/8" header flange, 1/2" T25 stainless flange, and 304L 1 1/4" schedule 10 tubing, all merged and honed smooth. Here are two pics.