I would think that with a P-40 you would already have adequate cooling... it has a giant air scoop under the nose and a small one on top, plus the exhaust stacks if you can open those up too. That should provide plenty of airflow if it can be vented properly.
I have also experimented with putting small "fan blade fins" on my motor bells, made from aluminum tape (or cellophane tape), that help create a vortex of air around the motor to dissipate the heat. It does actually work... I have done it to several planes where the motors sit behind large dummy radials that block the air and they went from running hot to running cool, and didn't seem to affect balance or RPM.
For even more fun, you can mount a stick or something that "interferes" with the fins to create a fake engine sound.... much like the old "card in the spokes" trick that we did as kids with our bicycles. I had that happen on accident once (dummy push-rod slipped through the dummy engine bell) and my plane sounded like it had a 2-stroke in it.