on stock frames 75 had the seat hinges brackets on the oppisite side of 77/78 and the oil tanks have different cap than 77/78. 77/78 side covers did not have a hole for the cap to stick out. i believe the earlier f models had that same thing too. i have never really looked to see if the different oil tanks will interchange between the two,if they do the side covers would also. the later front ends are different also in the design of the triple trees and the gas tanks,,the front mount should work,,i am not certain on the rear,, a tape measure would determine that.
whatever you wanted though to do though,,it would only take is some minor welding if not correct. good luck