66fs, I spoke with Inline Tube today about availability of the '67-'68 prop valve (they actually show it in their YouTube video) , and it mounts inline after the distribution block, I assume that is the one you are using. He said they are no longer available, I would have to find a used one or go with the adjustable type. I think the factory one is what I (ideally) would like to find. It would have the preset rear brake bias setting built in.
Edit: looks like Mega Parts carries these:
Proportioning Valve 1967-70 A-Body Disc Brakes 1967-70 B-Body Disc Brakes 1970-71 E-Body Disc Brakes, Brake Junction/Distribution Blocks, 129-RBV004 Reproduction, NOS and Used Mopar Muscle Car Parts