The '77-'78 F and K models use two completely different forks/brackets/calipers. Sadly, they're not interchangeable.
I have a caliper/bracket from a '75 K model if you're interested in it.
The caliper will need to be rebuilt, and the bracket is missing the adjuster hardware. But both the rebuild kit and the hardware are easily sourced from BikeBandit/Parts-N-More/etc.