I don't think performance oriented machine shops mill intake manifolds anymore, that was something we did in the disco era. Today the manifold mating surface of the head is milled instead! This way the modified gasket surface stays with the casting that created the need in the first place and the intake manifold can be re-used on other engines without fitment issues. Don't go cutting that Blue Thunder manifold!
Like you today, we didn't have bore scopes in the disco era either ... we actually got by without them. You can trial fit the parts, mark the manifold's location on the head with a felt tip & then remove the manifold and meaure downward from the felt tip lines with a machinists ruler. That will give you an idea where the manifold is at in relation to the head.
It worked for me ... when I wasn't dancing the hustle that is.
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