With a proportioning valve, you can run whatever you want.
With that being said, the bigger issue is c-clip axles and fixed piston calipers. You'd need to either convert to a bolt in axle or move to an aftermarket rearend to run them without significant pad knockback. The issue with any fixed piston caliper is that the rear axles can move in and out a bit with the c-clips. With a sliding caliper, it doesn't matter if there's a little play in the axle. But with a fixed caliper, you'd feel that as the brake pedal going to the floor and a momentary oh **** nothing is happening reaction.
But if you did convert to a bolt-in axle and ran a Wilwood Proportioning valve you could do the Wilwood 4-wheel kit with either 14 or 13" front rotors and 6-piston calipers and 12.8" rear rotors and 4-piston calipers...