Overheating is a complex issue and can have many causes. The thermostat might be the culprit. This can be removed and easily tested. You could also have an inoperative cooling fan, due to defective relay, bad sensor, bad fan motor or blown fuse.
You could also have a clogged radiator, which again, can be flow tested.
You could have head gasket issues.
Diagnosing overheating requires a step by step diagnosis by which items are eliminated by testing. Certainly first steps would be to check if the electric cooling fan is working. If it seems to work properly, then test the thermostat. If that is opening correctly, then I'd suspect the radiator, on an old car like this. If there's no sign of clogging, then I'd test for a bad head gasket.