Developing Flight Models?

[Pursuivant]

Creating the P-51A Apache would basically have to be a new airplane, due to different engine, addition of dive brakes, different nose (due to armament and engine changes) and other factors.

Treetop's got the details right. But if you're looking for a good place to start, look at changes to internal armament - especially changes that don't require any changes to the external model.

All internal armament changes do in terms of FM is alter mass and perhaps Center of Gravity (and IL2 doesn't do dynamic changes to CoG, which makes things easier).

External muzzles might slightly alter drag coefficient, but complicate modeling more than FM.