Take it to a machine shop and let them true it on a belt sander. It is cast and it will not bend. I always bolt the intake and exhaust together and have them surfaced together. Otherwise you will have exhaust leaks and you will gradually burn away at your exhaust manifold. It is common for inliner manifolds to be warped and it is very common for people to either never measure them or just use them and then they wonder why they have vacuum leaks and exhaust leaks. Having them trued has been an almost standard thing for the over 50 years I have been working on these old machines.