I have a Buick straight 8 engine and it's manifolds are even longer than the Chevy ones and very prone to warping. After surfacing the manifold to head surface several times the alignment rings were too wide for the manifold to seat against the head so I shaved a bit off of two rings for alignment, threw the gaskets in the trash and used high heat RTV RED silicone. Been at least twenty years with no exhaust leak. The RTV silicone seals with adhesion, not compression, so all surfaces have to be operating room clean and let to cure at least a day at normal temperature. I left mine cure a week. Picture is when sealant was about 15 years old and is starting to peel around the edges but still not leaking.
http://s32.photobucket.com/user/coilover/media/Straight%208%20a-c%20mount%20001.jpg.html

Evan