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1965 Merc 3.9 overheats at idle

I bought the motor recently. I've rebuilt the fuel pump and replaced the points and impeller. I cleaned the carb and replaced the fuel filter. The motor runs fine at high speed but at slower speeds the pee stream gradually gets hotter, begins to steam and eventually quits causing the motor to quit.

I'm guessing that the gaskets around the copper cooling line are leaking but I don't know. How much of a job is it to get to the top end of that copper tube between the water pump and the head?
Is there another possibility for these symptoms?
 
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