I think the Mustang looks fine with the square wingtips. Now when the Spitfire appeared with the squared off wing tips, that just did not look right.
As for the engine in the P-51, that would be the Rolls Royce Merlin, which also produced engines for the Spitfire models.
I guess Bish is a bit more patient with me, and did not seem to mind when I intially had thought of doing a Seafire, which is just a naval version of the Spitfire.
regards,
Jack