I like the idea. I was thinking of a similar mod that would use an aluminum hood with counter-flow pathways in it, so that the heat would rise giving better natural convection while stopped. I haven't done the math, but most radiators are specified based on surface area so your under car area would need to be around that range to compete with the original radiator. Another idea to allow the use of a radiator while reducing drag would be to place the radiator is such a place that the inlets could be 'NACA duct' shaped. I think I'd want it on the bottom as part of a belly pan for center of gravity and beauty reasons.
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