If you have sealing problems already likely from warped manifolds I'll add another step.
Originally Posted by tclederman
Do not fully tighten the two manifolds to the head - just light-snug tighten them with no gaskets between the manifolds and the head. Then, fully tighten the intake to exhaust manifold with one gasket between them. Now take the assembly to a machine shop to have them ground flat while bolted together. Then, put the manifold-to-head gaskets in place and tighten the manifolds to the head.

•1951 GMC 250 in the Project Journals
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•1952 Chevrolet 3800 pickup
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