You can run any frontend from 73-91 on K5's. You just have to have all the frontend parts including the core support and they work.
So if you have all the parts from a 73-80 truck you can run the round headlight square fender setup.
You can also run the 87+ frontend. I believe fenders and hoods are the same, the headlights and core supports are different.
As for singular parts that will interchange with your current setup, it should work with any parts from 81-86.
Harley