Fast-moving coolant may be the problem
In regard to a reader’s letter about his problem with a 1966 T-Bird overheating at 70 mph, it seems as though he’s covered his bases trying to find a solution. However, his problem may be due to the coolant flowing through his cooling system too fast. If the coolant moves through the radiator too fast it does not give the fan enough time to cool it down. When I raced Flathead Fords in the 1950s we ran in second gear and used a rear end ratio that let us use maximum engine rpm down the track. We used washers to reduce the coolant openings in the cylinder heads and machined off every other fin of the water pumps to reduce the rate of coolant flow.
You are quite correct that an engine may run hotter if the coolant is flowing too quickly. This is a good example of a situation where more is not necessarily better. Thank you for sharing your thoughts and experience.