Actuslly, if you convert to rear disks, it would be strictly a bolt on thing. Adapter kits and lines are readily available.
fix the brake issue now, do the rear disk and modern MC swap at the same time. Will make MC life easier because there is a bolt in brake line adapter kit. One less MC to bench bleed and deal with.
technically you could use it now with an 87-93 MC if you plug the bottom port. Seems a little...hack though. But it could work. But then it all goes in the trash when you do rear disks becUse that’s a different MC
I'm starting to think there's a blockage in your proportioning valve.
Which caliper is it again that is freezing up? The two front lines are split. One should go to the rear, and one comes off the bottom, rear port of the PV. I'm thinking one of these ports might be clogged. When you step on the brake pedal, you have more than enough power to force fluid past a blockage, but the natural wobble of the rotor might not be enough for it to flow back.
The problem is the fitting layout of the PV is different on the 87+ cars as they are a 3-port MC with one line direct, and a 2 port PV with 2 lines in and 2 out. The 80-86 PV is 2 in and 3 out. I also am not sure if the fitting sizes are the same. I believe the 87+ is a mis of 3/8-24 and 7/16-24 inverted flare.
The part number for the 80-86 PV is E0ZC-2B328-AA, You can either try to replace it, or remove it and rebuild it like the link I posted several posts up.
If you've replaced the MC, the calipers several times and tested the lines, it's the only think that it could be.
One more question. When you bleed the front brakes, do they both bleed at the same flow, or is the sticking wheel a little slower to bleed through?