Being your Olds engine was not a marine engine and out of a boat and if it was salt water cooled you better check the water pump impeller..The marine ones are sometimes stainless while the auto version is not..Salt water erodes non stainless parts pretty fast..I would never use a marine engine in a car if it was straight salt water cooled. The salt has taken too much material from the cast iron and raising the temp from the usual marine 140 degree stat to 180 or 195 can create a lot of heating problems due to even more break down of the cast iron plugging up the rear of the block..Besides having the cylinder walls too thin..But if it was freshwater cooled.(with an exchanger) and run in fresh water that could be a different story...OK...The heat exchanger is like a radiator..It has two parts..One half is sea water ..the other half antifreeze 50/50 along with the block..The sea water cools the fresh water in the same sense as the air going thru the radiator core..All this might not have even the slightest bearing on your problem but I had time to kill and can't seem to stop writing....I would pull the water pump and have a look see.. Good luck..Al..