I'll tell you what a very active P-51 pilot told me. I asked him how difficult it was to fly a P-51. His answer was, "How hard can it be? It was designed to be flown by 19-year-old kids with 250 hours in a T-6." What he meant, as he explained, is that it is NOT a SIMPLE aircraft to fly but, if you have 250 hours in a WWII advanced trainer, it also isn't too difficult as long as you follow the rules of P-51 flying.
I take that to mean that I, who has hours in Cessna and Pipers, would likely kill myself trying to fly one by myself without any type training, but that it isn't especially difficult if you have some decent experience in complex (meaning constant-speed prop and rectractable landing gear) aircraft with WWII fighter-type wing loading and some decent horsepower. I have leaned over a friend's shoulder and played with the control stick while in the back seat of a P-51, and it flew EXACTLY as I imagined with would. We were at cruise power.
So ... not much help, but also not totally unknown.
Funny how we see the same thing with 20 series Learjets, MU2's. It isn't too difficult as long as you follow the rules of insert name of airplane being flown here flying. (And stay proficient. Not just current)