the disc brake proportion valve is different than the drum valve, so you need to swap them. i think the 73 and later items are different and wont work, i know the disc items are different. you may can find used parts from another 70-72 a body car, like a monte carlo.
on my 72, i got new disc items from ebay and they worked fine. i wanted stock type so i could use any pads, and not need special order ones. kit came with spindles, backing plates and all I needed. about the same price as used I had bought before for the 70 car I have. I think inline tube also has a correct proportioning valve available. I did buy a used one of those from a disc brake car.
hope this helps. good luck.