Chad,
Sorry to have to correct you, but there is no way that a spinner nut is going to look scale on a P-51. All P-51's had a spinner on them, see the attached picture. Since you brought it up, to make this plane look scale he's going to have to put a spinner on it.

khoat6,
To get it to fit with a spinner, the easiest solution for you is to sand back the front of the fuselage. Since you motor is already mounted in place you need to sand the front of the fuselage off until the spinner ring, spinner backplate, prop, and spinner will fit properly. Please see the second picture to see what I am talking about.

Hope this helps

Ken