Why not just get the P-63D version with the M9?
"The M9 was a very different weapon. The large cartridge case gave the same HE round as the M4 a considerably higher muzzle velocity. One of the types of ammunition available was a 752g armour-piercing projectile with a muzzle velocity of 930m/s, and at a distance of 460m this penetrated 6cm of armour plate. At the same distance the M4 could penetrate only 2cm of armour. It is obvious that the M9 was much better suited for ground attack; but it seems that the only aircraft in which it was ever fitted was the one-off P-63D."
*edit*grr, never mind. They only built one D model and then stopped because performance was about the same as the P-51D already flying.