Intake manifold won't seal - How do I check it ?

Heres a question and its something Ive never read any talk about. When you replaced the heads how thick of a head gasket did you use ? If you went from a stock .028" to a .040" fell pro I would think that would cause the intake to need a gasket around .012" wider then stock also. Ive never heard of this being an issue before oddly enough.