Just a thought, the Mustang was originally commissioned by the Anglo French Procurement Board. If the commission had been let earlier or if France had not fallen so quickly, you might see Allison-engined Mustangs in French markings. 'Twould be something different.
Regarding engine changes, NAA looked at the Continental IV1430 and R-R Griffon as well as Allison's initial two-stage supercharger propsoal amd all of them were rejected as they all required major airframe changes. NAA's managemnt was especially leaned on regarding the Allison engine since, at that time, NAA and Allison were both owned by General Motors and the Board of Directors wanted to keep teh Mustang engine in-house if at all possible.
An excellent book covering the Mustang from the founding of NAA up to beyond D-Day is P-51B, How North American's bastard Stepchild Fighter Saved the Eighth Air Force. Apparently, there was a prejudice against the Mustane in USAAF Material Command from General Echols on down (seems he wanted NAA to take over p-40 production instead so Curtiss could retool to build the production P-46, which never passed the XP-46 trials. Echols was also a major force behind the P-75 as a long-range escort for bomber streams over Germany and he was not overjoyed when NAA managed to develop the P-51B et al. to do the job.
