First, you will not gain by putting disc's on the rear!, It's more for looks unless you plan on doing some road course driving! Same with drilled or sloted rotors.
Drum brakes will Always have better stopping power over disc's because of the locking feature of the shoes to the drum, versus clamping of a disc.
And yes drum are better for drag racing and they will not fade for the time you use them. and they will stop you better.
If you want dics on your S-10 rearend you can get them from a Bravada, most (not all) had them or get them from a '98? and newer.

A proportioning valve is not what you want, you want a combination valve . A combination valve has a proportioning valve, A metering valve, and a pressure differential valve all built in one.
A Metering valve puts the pessure to the rear brakes first untill they just basiclly touch the drums or disc's, and then allow full pressure to the front for a second and then puts the pressure to the rear, because the rear take less to lock and it sets them ready and when your braking with the front for that instant, then the rear can tighten down without locking up.
There is 2 basic proportioning valves, one for Front disc/rear drums is set for 70% front brake, and 30% rear brake pressure.
One for disc front & rear is set for 50% front brake and 50% rear brake pressure.
You could use a adjustable one to fine tune your brakes if your going to road course or circle track, or if you don't have the right master cylinder bore sizes and you lock the rear to easy. you don't ever put on in the front to stop if your locking the front easy! Get the right MC!
You will NOT! have a volume problem if you get the right mc for your setup!
The pressure differential valve is a valve that will move from center to a side and and close off a side if you get a leak and trip a light, and allow you to still have front or rear brakes to stop with a leak in the line with a dual master cylinder

There 1000's of people out there that have done this change to rear disc's and have no problems doing it.
I don't know anyone that has used a pressure gauge to check what they have though. Not saying nobody does it, I just never seen or heard anyone do it.

I do not like the brake light set up that Wilwood has, because you get a leak in the rear brake portion, you loose brake lights! It's a pressure switch and I only use mechanical brake switchs.

The weight of the vehicle comes into play with the size of brakes you use, not by the pressure!

You would think the engineers at Ford all got drunk one day at work and came up with the 3 line master cylinder on the Mustang from '87 to 93. They thought they needed a 3rd line from the master cylinder to go to the left front brake only! the rest went to the combination valve

Pat