This is the side of the header which faces the engine, which is hard to see from above or below when attached to the head. The arrow is pointing to where the crack is, which is along the weld joint, then goes across to the right for about 1".
Close-up of the Crack
Here's a close-up of where the crack is (see arrow). You can see that there's some rust surrounding the horizontal part of the crack. I was able to widen the crack by pulling the headers apart by hand, which showed how bad the crack had propagated.
