This is one of my most asked problems to solve. There are many reasons you could be experiencing over heating, besides all the obvious things like a bad or plugged radiator. Here are a few other things to check:
Be sure the water pump impellers are very close to the inside of the timing cover. They should just touch without the gasket. Ttiming covers are sometimes damaged by a worn out water pump in the past. In 1961 the AC cars started getting 5 impellers instead of 3 and we can custom build all 1956-1960 water pumps with 5 impellers.
Ignition timing is very important so make sure it correct and vacuum advance is in good working order. I have had the outer ring of the 1957-1966 balancers spin on the inner section so you think the timing is right but it is not. Buick also started using a smaller water pump pulley in 1962 on AC cars, those pump more water by over driving the water pump and fan to 1.2. These smaller pulleys must always use a clutch type fan and a fan shroud. It is also possible to pump too much coolant through the radiator too fast, No thermostat is one way that can happen or too small of a radiator is another. Remember that all the air that goes through the radiator MUST get out of the engine compartment. Another very common problem is rust plugging up the radiator AFTER a rebuild. The new hot tanks DO NOT remove chunks of rust, you think because the block and heads were "tanked" and cleaned all the rust was gone. It has happened so many times to us we started putting in a radiator filter inside the upper radiator hose. We use about an 8" square piece of pantyhose (ask the wife first! LOL) put it over the upper radiator fitting, push the center into the fitting creating a pocket, the outer part the pantyhose filter goes on the outside of the fitting and upper hose slides one holding the filter in place. You will be amazed at all the rust chunks you can keep out of the radiator. Clean often and remove the filter before driving a long distance where it could get plugged up and fry your engine!. Aluminum Radiators DO NOT cool as well as brass/copper ones do and when the aluminum ones get plugged you can't just tack the tanks off to clean, they are welded together. Last thing is coolant, use 50/50, half distilled water. Coolant bottle is helpful to keep that radiator filled.