There are a few subtle changes between the wiring harnesses for a 76 cb550K and 550F. The main being the right hand controls, starter button and lights.
On the Super Sport K, they have a on/off switch for the lights. On the 550F, there is no on/off for the lights, they are always on until you press the starter button. When the starter button is engaged it breaks the loop for the headlight.
One could modify either harness to work on either bike but you'd have to understand the wiring fully. I personally did not like having to modify my 550K harness to fit by 550F bike. And finding a replacement 550F harness on ebay in decent condition at a price under $100 hasn't been easy. DSS does sell a brand new harness for $175, which is a lot, but might be worth it.
There might be some other subtle differences between the two harnesses like the horn power but that is the main difference.