Drag car brake/proportioning question

You should at least have an adjustable prop valve in the mix somewhere. The valve you show is probably a residual pressure valve, which maintains pressure in the lines for the rear brakes. That's typically only a drum brake deal, they do better with a residual brake valve because they stay "staged" so they act faster. With disks a residual valve isn't necessary unless the master cylinder is mounted below the level of the calipers/lines, and should only be a 2 psi valve if that. With too much residual pressure the disks will drag.

The adjustable prop valve is nice because there wasn't any factory 4 wheel disk system for these cars. The 4 wheel drum cars didn't have a proportioning valve, just a distribution block. But the brakes were designed/sized to work without one, so the front rear bias didn't require one to be balanced. With an aftermarket disk set up the you may not be using disks and calipers off the same car or application, so there's no guarantee that the front/rear bias will be correct for your car. An adjustable valve would let you reduce pressure to the rears so you don't end up *** end first with a hard brake application.