Do you need a proportioning valve on disc/disc? I would say there is no easy answer to that. You have a 3/4 ton. What did the rear discs come off of? If you have the same braking ability, front and rear, the rears will lock up you when you jump on them. You don't want either to lock up, but if the rear does first, the truck will swap ends.
Residual valves?
http://www.mpbrakes.com/technical-support/configurations.cfmOne more little click.
http://www.mpbrakes.com/uploads/documents/residualvalves.pdfBooster?
http://www.mpbrakes.com/technical-support/configurations.cfmAnother click.
http://www.mpbrakes.com/uploads/documents/optomize.pdfMy '67 3/4 weighs 4600 lb. I used a booster from a '80s 3/4 Chevy van because they can be mounted flush to the firewall. The pickup boosters sit out about three inches. A 7" might be on the small side.
beltfed, Going from 1 1/8" down to 7/8" would increase the pedal travel. I would have looked at the pedal ratio
http://www.mpbrakes.com/uploads/documents/pedalratiopdf.pdf and then maybe looked at a bigger booster for a car that big. If it works and the pedal travel is OK, then good enough.
I only used MP's web page because it is easy to find what you're looking for. They don't really have answers, but a good place to start and to see what you can expect with different changes.
