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Air flows pretty fast through the radiator at speed. Its depends on fin count too. Those crank mounted fans turn slower but have more blade surface area than belt drives. I've not heard too many complaints from 39 deluxe or 40 Ford owners about problems with overheating as long as the cooling system is clear. I don't know a lot of 39 or 40 Merc owners so I don't know about them. They should be about the same as post war cars as far as reliability.