I’m rebuilding my 1971 350 Cutlass motor. Purchased a used Edelbrock performer intake. The mating surface is flat but when I did a dry fit the front and back sealing area is not even. I plan to use gasket maker there instead of rubber seal. Do I have a bigger problem with the manifold and do you see a problem with using gasket maker in the opening?

 

The critical sealing surface is at the heads. So long as those are both parallel to the head intake surfaces, the mate to the block doesn't matter. Use enough RTV to ensure the gap is filled when you set the manifold down.