I had a '53 F250 with the V8 flathead and it overheated at highway speeds. It appeared to be clean internally, observed when I replaced the thermostats and water pumps.......still overheated. Radiator cleaned.....still overheated. I went and found an "older" mechanic at the Ford dealership and he was happy to chat about flatheads. When I told him what series truck it was in he said that with the 4.86:1 gearing it was moving the water through the system too fast to cool it. "Cut a piece of brass stock to the size of the thermostat and drill a 1/2" or 3/4" hole in the middle", he said. "Replace the thermostat with this". Guess what, cool as a cucumber on the road then. Sometimes experience just can't be beat!