Chad, the documentation I have confirms what you say.

It says "the installation of this tank (the 85 gal fuselage tank) would be the factor that resulted in high speed porpoising and would cause the later replacement of the fabric covered elevators with metal covered surfaces and a decrease of the tail-plane incidence." That reference and other sources say the original incidence was +2° which was changed to +1/2°. (sce: Mustang - the story of the P-51 fighter by Robert Gruehagen)

Another source says that the change was made during late D model production.

In reading about the Mustang's development it's interesting that despite its remarkably short design period, it didn't have as many operational problems as other fighters with longer development times and all the problems were easily fixed. The basic design was sound.

Yet when it went into initial production only the wing had been wind tunnel tested. Other interesting trivia was that the Chief Design Engineer Edgar Schumued was a German immigrant who was not a degreed engineer. The Mustang was the first major project for chief aerodynamicist Ed Horkley. They and the rest of the team produced a fighter with the lowest drag of any prop driven airplane in during WWII. That and the fuel efficient Merlin are what gave it the longest range of any of the escort fighters.

Sure makes interesting reading!

Tony