Due to the high wing loading of a warbird such as a P 51 I would suggest you build in the flaps. Also do to the fact that it is a high wing loading warbird I would also suggest that you get yourself a 40 size trainer and learn on it. The P-51 is not a plane that is good to learn on as a matter of fact without flaps it is a real handful to land. If you try to fly it as your first plane the life of the plane will be measured in seconds ( http://www.youtube.com/watch?v=t1xysvXKVB8 ), it may survive if you have a real good instructor on a buddy box, but I would still suggest you get a trainer. The cost will be minimal if it saves your Mustang after all the time you put into building it. By all means if you are new to RC be sure to start with an instructor and add the flaps, the P-51 is heavy and lands hot.
Good luck!