I took the easy way out. I had an old exhaust manifold kicking around that was badly cracked but had excellent threads. I took a very sharp file and used the corner to cut V grooves acress the threads, basically making the old manifold into a giant tap. Whenever I had to freshen up an exhaust pack nut I would clamp the old mainifold into the vise and using my big wrench would work the nut back and forth on the old manifold's threads. It worked great.