The A36 and the P-51 are to very different aircraft. Enough so that the designations remained distinctly different.
They do share some nifty looking comparisons when looked at in profile though... I can understand why it confuses most.
While what you say is true there may be people that come along behind us and think what you have said somehow disputes what I have contributed here so I will clarify what I mean.
The NA-73X project was acceptable to the British and the aircraft further evolved into the Mustang line (NA-91). The NA-73X line in its purist form becomes the P-51 and is further evolved into an attack aircraft as the A-36. From the very beginning the 'Plan name' of both projects is Apache. Furthermore the aircraft is first called Apache as it enters into US Army inventory. 55 of the NA-91 line (P-51s in Air Force terms) are retained by the Army for modification into photo reconnaissance and are designated F-6A. All of the pilots first assigned to Peterson Field Colorado were trained in F-6A Apaches which within the first month officially became F-6A Mustangs.
This is touched upon just briefly in the book "Mustang: The Story of the P-51 Fighter" by Robert W. Gruenhagen on page 60 where he writes (following the 'Oh S**t' moment I mentioned above):
At this time the Air Force accepted the Mustang and in an effort to establish its new mission and identity the name "Apache" was assigned to the P-51. By the time the "Apache" was received by flying units the designation had been changed to F-6A indicating a photo recon ship and the name was accepted as Mustang to conform to the standards being established by British use of the aircraft.More than 800 pilots would be trained in the F-6A and some of them would later fly the A-36 Apache in a photo recon role.
Also the A-36 was the first of the Mustang line that allowed for the mounting of bombs or external fuel tanks. The A-36 played a pivotal role in inventory acceptance and so it most certainly should be present in AHII.
