You can use the factory combination valve, which combines a fixed, one-size-fits-all proportioning valve along with some other features, and you can plumb an adjustable proportioning valve behind the combination valve for added adjustability of rear brakes.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)