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Google "Marine Carburetor Tilt Shim". I may be wrong, but it seems to me that CC intentionally had the angle built into the intake manifold rather than using a tilt shim like many other manufacturers such as Crusader used for carbureted inboard engine applications. That would be the Chris Craft way of the 1960s. You would probably not see that done today.
The good news is that there are tilt shims available for Carter AFB style carbs that could be used to keep the carb level with an aftermarket automative intake such as the Edelbrock Performer 390. I would probably use a tilt shim if I were going with that combination.