Personally, I say have the greenhouse-canopied P-51B and the Malcom hood as the P-51C, then we could have 30 skins instead of 15.
Do the same with the P-51D/K, do a slight remodel on the bubble canopy and give it un-booted blades on the prop and call it a K.
Both "remodels" would have the same FM as their counterpart, easy way to double the amount of skins without having to speed up the replacement rotation.