I have the same engine in my 1983 C&C and it runs perfectly. Now I
have to admit mine is FW cooled but I really don't think internal
corrosion is likely to kill the engine. More like bearings and cam
wearing out.
If you compare the specs for the 3HM and the 3GM you will see the HM is
actually a heavier duty engine, greater displacement, horsepower, oil
capacity, etc. You are better off with a maybe rebuilt 3HM than with a
3GM that has definitely not been rebuilt. When sailing offshore you
are more likely to have your engine die from gunks stirred up in the
fuel tank than from anything going wrong with the engine.