Engine overheating

I have a 1974 ski tique.   I have had about 10 years. I have had engine rebuilt. But it still overheats. The manifolds are likely original. My mechanic thinks if we remove the gasket that restricts the water flow between the manifold and the riser. I won’t have the overheating problem.   There is not a way for me to post what this looks like. I can send pic by phone or email.   Cronedm@gmail.com 2055349321. Is there a reason the gasket between the riser and manifold restricts water flow? What’s the purpose of that type gasket there? TIRED OF BUSTING OUT ANOTHER THOUSAND 🙃

May I have a regular email or cell I’ll send pics of the engine and gasket.    I also have pics of the new manifold my mechanic wants to hold off on installing until he runs it without gasket. We are waiting until the bushing the prop shaft goes into to come in it was wallowed out and I was getting a vibration from prop area. It’s always something.