The F grab bar won't work the 77-78 K model seats. Will need to change that, too. If I recall, the F tail light housing gets hit too. So, you might need the rear fender and tall light for the swap, too.
The seat for the 77-78 has more comfort for the passenger than the F seat. So, I'm told by passengers.
The early k seats will fit to frame, too. But, interfere with the bigger F tank. Early tank fixes that issue. There is still a grab bar style change to go with the seat to consider.
Cheers,