68 Nautique 390 ford overheating

Paul,
Pictures would be great. It does sound like your 390 is set up with a closed cooling system similar to a car or truck. On a marine engine, it's commonly called fresh water cooling except the closed portion of the system is cooled by water instead of air. Fresh water cooling on a marine engine is typically added when the engine will be run is salt/brackish water and or if there are aluminum components on the engine in contact with the cooling fluid.. The closed portion can be cleaned just like a car by using one of the radiator flushing products found at the auto supply. I suggest the flushing but first as Chris mentioned open up the RWP (raw water pump) and check the impeller. The RWP will be a pump added to the engine to bring in lake water. Follow the hose from the thru hull fitting on the bottom of the hull to first belt driven pump. BTW, there should be a T stat. Maybe a PO was a hack and removed it as a back yard crutch for the overheat problem?? Check all the hoses on the suction side of the RWP for general condition and tightness at all the connections since the RWP may also be sucking air.

How have you determined the engine is a 390? A 352 would be more common in a SN of that era..

Welcome to CCfan. Yes, picture would be great plus tell us more about the boat. Have you had it very long? There's also a 1st generation SN registry thread that you shound check out and contribute to. 1st generation SN roster thread Besides the roster, a diary entry is always welcomed.

Have you checked the raw water pump? The circulating pump may be doing good, but the RWP needs to bring in lake water at idle.