I can't question your opinion re: the pilot's spine, but the environment in the P-51 is not conducive to survival so much as it is to success. I certainly don't criticize your experience in the F-4.
Here, there is a possible transient load of 21g. Almost twice that your buds survived. He was ok before the Roll, we assume. There is no provision for helmet anchoring a la NHRA, and the straps are less than useless in regimes outside their design, which is mostly for straight and level. If you pick up the PITCH changes, the distortion of the airframe, the TW loss of stow, the Rolling moment, and the YAW, then consider he is wearing coveralls, tennis shoes, and sports a brain bucket built for visibility, I don't like the chances for his spine to survive.
But. It is moot. Grey is enough to put this aircraft into the asphalt. Not even grey, for that matter. The stick is long, and wants deliberate and accurate movements. It is alive. Moving it in 3G is not without its challenge.