I don't think so. The tank and seat were different on the 77 and 78 models (K3 in the UK).
You could fit a seat/tank pair as the tank mounting is the same - but you'll probably have to move the seat hinge.
My avatar (left) shows my 500 in 1978 with a 550K3 seat and tank. The tank slipped right on but the seat needed mods as the hinge was in a different place.