You have be careful with CG on a plane with flaps. You can't move it back like you can with other non-flapped airplanes. First, flaps decrease the stability margin because wing lift is increased. Then, the flaps can decrease stabilizer effectiveness even further, so always start at 25% MAC. Third, with an aft CG, the plane will pitch up with flaps, perhaps becoming untrimmable, even if still stable. I haven't built the Mustang, but the CG on the Sea Fury is in a very small range.