A note of caution:
When you weld a header flange, bolt it or securely clamp it to a known flat surface and keep it there until it has completely cooled. Welding can distort the flange and cause you all kinds of sealing woes.
You can also use an old junk head to bolt it to. The idea is to keep it flat and true throughout the repair process; because if you warp it, you need to have the mating surface re-flattened either by surface grinding or on a large table/belt sander.
