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Note that the later P-38's added a lip to the radiator air intakes to separate the boundary layer. And the later higher powered engines with the better intercooler still used the same size engine coolant radiators as did the earlier models - they just got more efficient use of the radiators.
I wrote the Duncan's Hot Rod story and Ward told me that they were the same size radiators, but moved further out to get them away from the boundary layer. I believe him.
That tube and fin radiator is a post war replacement. All P-38s had bundled tube radiators.