HiCustomerand welcome to JustAnswer,
The thermostat is located beneath the alternator assembly, near the water pump. I can provide a diagram if you'd like.
From your description, it is possible that the thermostat is not opening enough for the coolant to flow through the radiator to cool off. I have repaired similar issues like this and the thermostat was the culprit.
Although, your description may also entail that a headgasket maybe on it's way. Damage could've been done when the car was driven with a cracked radiator. Those older composite headgasket doesn't take much to blow if the temp. range was in the red zone.
I would suggest to replace the thermostat first and pressure test the coolant system to see if an external leak is present. If the pressure drops and no external leak is found, then you may have a blown headgasket also. But, like I said, replace the thermostat and go from there.
I haven't recieved a response from you in awhile. Was there any additional questions you'd like to ask?