PS Gear Box Rebuild

The k member difference has to do with sway bar routing. The 73 and later disc brakes and steering linkage will work with the earlier K member. Either steering gear will bolt to either K member too.
If you have the 73 and later K, you gain the fail safe spool type motor mounts, and a sway bar routing through the middle of it. Why not use it?
If you need to retain the 68 K member, and add the 73 steering gear with its linkage and the disc brakes to it, you can do that. You would still need the earlier sway bar routing around the front of the K.