Another food for thought. While researching for parts to asemble my headers, I came across an interesting article on the Stahl Headers web site. They gas weld their header joints and braze the flanges onto their headers. They say that gas welding is self-annealing and the brazing allows for a larger fillet, which makes the joint more durable. If the headers are mig or tig welded, the headers have to be properly annealed to prevent premature failure.
www.stahlheaders.com