I'm not that 'educated' in the various detail changes on the P-51D, but the kit seat, and the one in the drawing, look more like the earlier, P-51B seat, although I think that early 'D' models used the same seat. The later seat was more 'rounded', although both were designed for use with a back-pack parachute - maybe Bill can add more.
Certainly there would be no room for a cushion (or dinghy pack) on a WW2 P-51D, due to the back-pack parachute.
I have an account in a book somewhere, describing how the use of a back-pack parachute saved the life of a pilot who's P-51D broke-up in mid air, after being hit by enemy fire.
He found himself in mid air and, not realising he was still strapped into his seat, pulled the parachute ripcord. The 'chute deployed from between his back and the seat, and he landed safely in a shallow pond, still in the seat.
Had the Mustang used a seat-type pack, this could not have happened, and, not realising he was still in his seat, the pilot would (probably) have fallen to his death.
The inclusion of the dinghy pack is probably due to the kit/accessory manufacturer using a current restoration as a pattern - something which happens far too frequently, without the company concerned checking details - the Airfix 1/48th scale Bf109E is a prime example, with the wrong canopy and fully extended oleos, due to using the 'jacked-up' RAF Museum example as a pattern. This aircraft lost it's original canopy, and a 'G' type was fitted for display' The oleos are fully extended as the aircraft is now displayed supported on jacks !