Overheating

The timing being advanced too much could cause an overheating problem.

You should have a shroud, it will direct the airflow where it needs to go.

Check your lower radiator hose, they are prone to collapse when they get hot. Get a new one with a spring support in it.

You should go out (when the engine is cold) and remove the radiator cap, then start the car and let it get hot. If you see the water moving around after a few minutes, the water pump is ok. If not, it could be a water pump (doubtful) or a stuck thermostat (How did you check yours?)

If the water pump is bad, 99% of the time it will be dumping antifreeze all over the place. If you have a bad head gasket, you will be blowing white smoke out the exhause or possibly be dumping antifreeze into your oil.

Back to running the engine.... if the water is heating up and is flowing through the radiator, it will expell an air bubble after a little while. When you check the antifreeze level (cold), be sure its about 3/4" down from the top, as it needs room to expand. An overfilled radiator can lead to overheating. If everything else checks out, try some Water-wetter, available at many parts stores. It will drop your radiator temp by about 20 degrees.

Finally get a shroud. I know I already said it, but its worth saying twice.

Most also miss the fact that the timing being too retarded will cause the car to overheat more than being too advanced so you need to verify that your advance mechanisms in the distibutor, both mechanical and vacuum, are working properly.

I'm bought a shroud for my 66 and I'm putting it on tomorrow.

I'm not sure if you don't have one. So just in case you do NOT have a fan shroud. I'll post back here and let you know how mine help my overheating.

My only real overheating problem is from the 4.11's and 4-speed on the highway at 70 for more than 10 minutes. I'll get back at you though!