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How familiar are you with setting the timing on a Model A engine? If it is set too retarded it will cause overheating and no amount of cleaning etc... will rectify its running hot.
Prestone radiator flush is targeted at hard scale type problems. Grease from too much lubrication of the water pump might be causing issues.
Try flushing the system with Simple Green. I would remove the radiator, plug the lower outlet and over flow tube and put 1 gallon straight Simple Green in it. Cap it and slosh it around. Let it sit for a while and slosh it around again. Do this for a couple of days and see what comes out. You might be surprised.