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My money is on plugged passages. I had a 1978 'Rude that had a similar issue. I pulled the heads and cleaned everything. Replaced the gaskets, thermostats, impeller and lower unit seals and oil. Turned around and sold it to a friend who still uses it today.

Most O/B motors that I have seen of this type have a pretty simple thermostat block. It's a few bucks and a half hour to swap thermostats and gaskets out. I do believe that the water would still be pumping, just not being directed into the passages if your T-stats were failing closed. I really do think you need to look at your water passages.