What models are you talking about? I think all frames but the Hondamatic models changed from 1976-1977. On K's the frame from the backbone back should be different (seat mounts, rear peg mounts, rear fender mounts, etc). On the F's the lower rear section of the frame changed to accomidate the new foot peg brackets and exhaust.
Mechanically speaking you should be able to change over almost everything (motor, suspension, etc) although some of the wiring may be a little short. Since you want to use a 76 frame, why not get the earlier bodywork (if it is a K bike) and run it that way. If it is an F you should be able to use your existing tank and seat.