I would guess that if skins were accepted from the end of wwII on - then a skin from 1998's reno air races would be correct then also? I would imagine HTC wouldn't accept a skin from after the war.
There are tons of pictures / profiles of p51s that would be fun to skin of the p51. The only p51s werent just the Big beautiful Dolls and Checkertails....
Also try books instead of just the internet......
if there's a 1/2 price book store near you - great deals on older books with tons of info, color pics.
Squadron Publications make very good books for research.
And You can even check out aeromaster decals....they've done the research on the plane - and give you the color scheme and ref. pics usually.
Good luck skinning and have fun