I had a stock exhaust manifold crack on a 2.5 4 cyl. S-10. The manifold was made like a set of headers. It was made out of tubing. It had developed a crack around one of the tubes at the flange that bolts to the head. I took it off and welded it back up with a self shielded flux core .035" wire.
It has held through numerous heat cycles, but it did not have insulation wrap on it. I've sold the truck to someone I work with and He is still driving it. He hasn't mentioned whether or not its still OK, so I'm assuming it is.