Proportioning Vavle for Disc Brake Conversion on a 1970 GMC C-20

[SonomaComa]

I purchased a 1970 GMC C-20 that already had a Disc Brake Conversion. I was told that he used the Spindle and Calipers from a 1972 Chevy Truck. The Upper Ball Joint had to be changed too because the taper on it was different for the Spindle. It does not have Power Brakes. I was wondering about a Proportioning Valve.
There is some sort of Distribution Box that the Brake Lines from the Master Cylinder go into and then one line goes out to the Front Brakes and one goes to the Rear Brakes. There is also one more Brake Line that goes to an old fashioned Trailer Brake Controller, the kind with a slide handle inside the Cab.
Would it be best to just hook the brake lines up directly to the Master Cylinder and install an adjustable Wilwood Proportioning Valve on the Rear Brake Line just for the Rear Brakes and run the Fronts right out of the Master? http://www.wilwood.com/MasterCylinde...temno=260-8419
What is the purpose of that Distribution Box? Was that stock on a 1970 GMC C-20?