@farmerman,
Well, yes you can. As a purpose-designed ground support fighter, it had an imperative for a long operational range. In the case of the original design, it had to be agile because it was a ground support fighter, but it also had to be in the air for a long period of time to be of any real use to a ground commander calling in air support. The original design produced an aircraft wich would spend six or seven hours over the battlefield, and still have the fuel to return to a base well behind the lines, out of enemy artillery range. No other single-engine, single-seat, propeller driven aircraft had the operational range of the P51--not even close.
As for clades, if one were to look at it that way, the Mustang and the Spitfire could not be further apart. The Spitfire's ancestry was in pylon racers. They had to be fast, and they had to be agile. But they only needed to be in the air for 15 or 20 minutes. Vickers-Armstrongs bought Supermarine in the late 1920s, and they continued to make pylon racers. The aircraft designer Mitchell at Vickers is often given credit for the Spitfire design, but he was actually using designs from the pylon racer. The wings were thin in cross section, which made them faster, and were relatively wide, to assure the necessary lift. The fuselage behind the wings was a short as possible--long enough so that turbulence from the trailing edges of the wings did not interfere with the tail assembly, but short enough to keep weight to a minimum, so as to retain high speed.
The Spitfire could not carry fuel in wing tanks, and it was limited to the small amount of space in the fuselage behind the pilot for fuel tanks. That was why it only had about 40 minutes operational time. Using the Rolls Royce Merlin made it faster, and because of the supercharger on that engine, it could still operate effectively at higher altitudes (useful for hunting bombers), but it still was a local CAP (combat air patrol) fighter.
The Mustang, with its Allison engine was only "underpowered" if one imagined a different operational use than it had originally been designed to fulfill. I do believe it was the RAF who suggested using the Merlin engine in the Mustang. This gave it much more power and speed, and because it was fuel-injected, it did not decrease its operational range, and it did give it the ceiling to be an effective escort fighter for the bomber streams going to Germany and Austria. (Asked after the war when he knew it was lost, Goering said: "When i saw the first Mustang over Berlin.")
This is what the Mustang looked like in the first production model:
The fuselage is much larger proportionally than that of the Spitfire, and the wings are thicker. This allowed for the wing tanks and the tanks behind the pilot which gave it its great operational range. These features were, in fact slightly exaggerated in later models to maintain high operational range.
As for clades, the Spitfire is a sparrow--the Mustang is a sparrow hawk.