No real engineering necessary for this one:
P-38K Taken to its final iteration, you'd call it the P-38R. Take the P-38K, swap in the Allison engines that ended up in the P-82. Add the FOUR blade paddle prop. Swap the 4 50 BMG package for 4 20MM cannons. No real engineering needed there, either.
Take the P-40N, and swap in the engine from a P-38L (turbo and all), and the four blade paddle prop. Put the 4 20MM package on it, or leave it with the 6 50 BMG package. All you have to do there is package the turbo, and raise the cowl around the prop. Not a lot of engineering there.
Put the turbocharger BACK on the P-39. Put a 6 50BMG package in the wings. Put a 20MM or a 40MM in the prop hub cannon. Add the four blade paddle prop.
Hang the 4 20MM package on the Goodyear Corsair.
