'73 up brake distribution block/proportioning valve question

The only difference between them is the sizes of the reservoirs, and early all-drum M/Cs had little hold-back valves in the ports to keep a lil pressure in the lines to prevent seepage of the slave cups.
If you have discs on the front, you have to remove that valve, to prevent the pads from dragging.
On an old Mopar the front discs are plumbed to the rear reservoir.
If you are running bigger tires on the rear than front, I have found rear hydraulic proportioning unnecessary; which can be achieved by varying the slave size, of which there are three to choose from.
The goal of proportioning is to prevent early rear-wheel lock-up, which usually ends up with the backend coming around, and wiping the car out. IMO, having more braking from the rear than the factory provided, is a good thing.