So, you are saying that someone has been into the intake manifold, there are signs that the gaskets were replaced….
I am suspect when I see things like this, always "wondering why"….
Generally speaking, an intake manifold leak/reed problem will be more evident at lower speeds. You mention the engine "runs OK" at lower speeds "but needs a lot of choke". Are you running the boat on the water, or in a bucket? There is a bogging problem at higher speeds, but the engine surges back to life when the throttle is reduced…
You mention that the carb/linkage is properly synchronized, but the symptom you mention makes me think something is off with it. Did you actually remove the high speed jets? Did you use the proper jet screwdriver?
How about the distributor belt? Is it properly indexed on the flywheel/distributor? If this is not set up properly, it might cause the symptoms you describe.
I guess at this point, I would probably start over from step one, can’t remember if you checked compression…
Back to your original question, it is surely possible that someone made a mess when they had that intake off. New gaskets are not expensive, just an investment in your time to pull it off and clean the gasket surfaces properly and check the reeds…Generally speaking, the new gaskets should be installed "dry".
I am trying to remember the story on this engine, but am assuming you bought it and it has never run correctly for you, is this the case?