I have a 58-62 235 in my '53 3600 and am having a bit of trouble with it. I have an exhaust leak coming from 2 spots where the manifold meets the head. I plan on taking the manifold to a machine shop to get it trued but I am still paranoid. I read that the new gaskets are half as thick as NOS ones. Are there any places selling the properly thick gaskets?

Also, do I have to separate the intake and exhaust manifolds to have it trued?