Proportioning valve for rear discs

Not a line lock ( that is to lock the fronts ) A front/rear adjustable proportioning valve to regulate front/rear bias.
FRom bias To Radial. Massive improvement in braking as well. After all it is the tire that grips the pavement not the brakes.
Mismatched components. Of course, they never came with rear disc so you almost can't help to parts not matching. Most kits use rear GM calipers.
I would say if you really want the "look" of rear disc and more overall brake performance, go with an aftermarket complete matched brake system. Wilwood, Baer etc...