Though I don't dispute what the military flight manual for a P-51 says, I expect that the priorities for military operation of the P-51 were different in the day, from the civilian operating priorities for a rare warbird now. The only time I have ever retracted the gear right after takeoff has been a very few occasions where I needed the improved acceleration and climb performance as early as possible. I certainly agree that there are times when the forced landing surface would be unsuitable enough that the airplane is better bellied in, but the runway ahead of you is not that!