kelsey hayes part and proportioning valve

If you are running big and little tires, forget that P valve. Make those rear brakes work as hard as you can, so long as they don't lock up first. If you need Proportioning back there, just swap to different size wheel cylinders.
I run the KHers up front with 235/60-14s, and on the back are 295/50-15s with 10x2" drums.; and NO Proportioning at all. I decreased the rear w/cs from 15/16 to 7/8 for 275/50-15s. But then swapped to 1" for the 295s. She's a great solid stopper now, with no tendency towards rear lock-up.
I also swapped out the 1" M/C for a 15/16", for a lil more accurate modulation.

As previously said; your only concern is that the rear end does not lock up prior to the front, which would send you into a spin; if you cannot back off, or if you panic I guess.

With big tires on the rear, and proper proportioning, you won't believe how nice the car stops, with those wimpy 235BFG fronts; it's like throwing out an anchor.