I think I bought a NOS one for $45 a couple of years ago to replace my broken one.  As I recall, the old threaded fitting in the exhaust manifold was rusted so badly into place that it had to be destroyed to remove it.  I put antisieze on the new one, but I have no idea whether that would be effective on such a hot part.

Why not just weld the crack. If u don't have a welder take the piece off and go to an exhaust shop. They should be able to help.