RE: Fast P-51
The 115 on the 14x14N will probably be the fastest but you might need 50% to get the speed you're looking for. Make sure you get a Hyde mount. The 1.15 has a lot of vibration and will shake everything loose if you don't.
I also wouldn't bother with flaps nor would I do barn door ailerons. They tend to cause negative yaw and are more susceptible to flutter. Look at Q40 planes and you will see the ailerons in the center of the trailing edge. Less chance of flutter but still far enough out to give some authority.