On the small block heads, I spot-face the area where head bolts pull down on the head square with the block after the angle mill is done. Unless you create a big change in the carburetor base angle, it shouldn't be necessary to modify the carb mounting flange or the exhaust pipe flange. You might want to test-fit the manifolds and use an angle finder to see how far out of level the carb base will be.
Jerry

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