If you're getting water constantly out of the overflow, there is a good possibility that you're getting compression gas into the cooling system due to a leaking head gasket, cracked head, or block. Especially if the engine was assembled back in the 60s and has never been taken apart for any reason. Head gaskets do deteriorate over time, and will start getting a compression gases into the cooling system via the leak.
If you remove the radiator cap and look down into the radiator with the engine idling you may see tell tale bubbles that continually come out as the engine is running. Another way to test for compression gasses is put the end of the overflow hose in a cup of water and look for bubbles in the cup as the engine idles.
If the leak is minuscule you may need a tester to determine if there is a leak. Link to using a combustion leak tester:
https://www.youtube.com/watch?v=vYz4Kl1IcSw
I would rule out that possibility first, then start looking at the radiator, fan clutch, and other parts that also could have deteriorated from old age. As has been mentioned, old radiators do scale internally over decades effectively insulating the cooling system from transferring heat from the radiator to the airstream.
Typically a radiator will lose 30 degrees from the top to the bottom if it is able to transfer heat properly. With your heat gun you should be able to check that for a problem. That's how I would troubleshoot the problem, hopefully this will help you find the cause of your overheating problem.