[THEFERMANATOR]

Thermostat would be my guess. There is a thermostat in each cylinder head in the top of it where the hose connects up at. Also the poppet valve is a big concern with overheating on MERCS as it controls when the t-stats get bypassed to allow for full cooling. The poppet valve is located on the lower stbd side of the engine and is very common for it to get salted up over the years and stick open causing overheating. Normally it will cause it on both sides though. I would start by putting in a set t-stats first and see if that does it. And also make sure the hoses coming off the housings are in good shape with no kinks in them and are free flowing.