Did he ever fly a B/C with the Malcolm Hood? If not, what is your basis for claiming different handling for the two?
Personally I don't believe there will be a significant performance difference between the bird cage and Malcolm Hood P-51B/C. The best way to add this would probably be to have duplicate gun packages that change the cockpit framing the way one choice makes the B-25C a solid nosed version. Not sure that AH is setup for it right now though.
it is you just need to make it a loadout option like you do the B25s. the 51B had the Birdcage once upon a time i dont see how it cant come back again. as in the B25C make it a 4 gun option Malcolm and a 4 gun option Birdcage for the 51