this part looks correct.
http://laurelmountainmustang.com/products/1968-1973-mustang-brake-distribution-block-2
it has the nut on the bottom that packs the prop valve.
the power disc and manual brake distribution block is different.
the prop and distribution block is all integrated the laurel hill photo shows the pressure warning sensor which shows if you have a leak on one side the system.
inside the block is the prop valve, which is adjustable and can get you into trouble. there is a heavy spring and various seals that bias the brake fluid to lock up the front before the rear drums. basically the valve only effects the rear drums, the front disc brakes bypass the prop valve and full pressure goes to the front disc. the rear is restricted and opens later which keeps the rears from locking up first.
there are rebuild kits that did not exist 5 years ago. internally there isn't much to the prop valve, if the body of the prop valve is in good shape you can rebuild the original. i rebuilt mine with parts from another valve as rebuild kits did not exist back then.
this is a rebuild kit
http://www.mustangsunlimited.com/itemdy00.asp?T1=1G3+01
90% of the time the original rubber seals fall apart or they swell and the prop valve jams and no fluid reaches the rear drums.
you pop the bottom nut on the block to open the prop valve, remove the sensor switch first. then pop the nut, and go slow the heavy spring inside will make the assembly want to fly out so be prepared then carefully remove the assembly and note order of assembly.
now when you go to change the seals you will need to unscrew the bottom from the top, you need to count how many turns you made to take the valve apart because you need to match it during reassembly so that the original OEM pressure is maintained to the rear drums.
you can mess with it and make the drums lock early or later just by turning the screw. this is all from memory about 10 years ago.
my issue was seals were contaminated and swelled locking the prop valve and not allowing fluid to the rear line.