Adjustable proportioning valve

IF you have the entire brake setup including the distribution block from a '73 then it almost certainluy is a combination block w/ proportioning valve.

You can find descriptions in the Master Technician's Conference booklets f
try 1972 Disk Brake Roundup vol 72-05
and then look in '73 for updates.
http://www.imperialclub.org/Repair/Lit/Master/index.htm

If you are using the 1968 distribution block, then you need to add a proportioning valve in the line going to the rear.

An original prop valve for '68 is not adjustable. If you want adjustable you can get one for an early Mustang/Corvette kelsey hayes system, or from Wilwood, Mopar Performance etc.