If you find the fan clutch is not the culrprit then I hate to be the bearer of bad news but... I had a '86 Ferd gasser and went through the same thing you are going through. It ended up being the transmission slipping. The trans oil runs through the rad before it goes to the air cooler. If it is slipping and creating a lot of heat, it will heat the coolant up.

On edit, when I say transmission, I am including the torque converter.