318 exhaust manifolds on a 360

I forgot to mention that it would also be advisable to use a set of inside calipers (see pic) or a deep throat ball mic. With these tools you can check the wall thickness as you cut so that you don't make any of the walls too thin which would cause a weakness and cracking.
When I remove metal from the manifold I always make sure that none of the walls are any thinner than the thinnest part of the factory casting. This way the manifold is at least as strong as it was intended to be.
Treblig