The F model foot peg folds up for kick start lever clearance when kick starting and has a dedicated mounting stud that is unique to the F frame - you may be able to install one to a K frame so that a F foot peg will fit though. If you aren't using an F model kick start lever the K model foot peg will work fine.
The F model brake lever pivot tube and pivot shaft are both much longer to allow room for the four into one collector and the brake lever is constructed from flat steel rather than tube to allow the kick starter to clear it and so that cornering clearance isn't compromised.
The F model kick start lever will not work with the K foot peg, it needs the F foot peg that has a fold up mechanism like the 400-4 has. I have found that a 77/78 K model kick start lever will work with the F model 4 into 1 fitted but scrapes the exhaust when kicking.
So putting a 4 into 1 from an F onto a K is possible but not by just bolting on the right parts.
John