Gpz has triple disks, and the front disks are slightly smaller (diameter and thickness) than the single disk on an LTD.
This means the brake caliper mounts at a different height on the fork (so the forks are slightly different). To swap it, you would have to make sure you can swap disks, and in order to do that, you have to make sure you have the type that is not mounted to a carrier. They mount directly to the wheel on the gpz and later LTD's. Also note, the 82 and later gpz's had a different bolt pattern for the disks, so those will require disks from some other model Kz in order to be large enough to use with the LTD forks, if there is even one with the same bolt pattern... or you can swap to gpz forks, but those are harder to come by.
The rear of the bike would obviously have to be retrofitted to take a disk brake assembly. It's probably doable, but it will take some welding and fabbing, or maybe you can swap an entire gpz rear swingarm assembly, and modify the frame to take a hydraulic master.
It's a lot of work considering an 18" Kz550A model wheel should probably bolt on.
It's just so much easier to wait for the right thing to come along.
