Proportioning valve

ok, not that my brain has been totally abused from all the above, let me ask a simple question and hopefully i can understand the answer. My 72 dart swinger 318 had factory brake drums all around. (build sheet info) it came to me with disc. conversion with what appears to be 1973 disc brakes on front. they have been on the car for a l o n g time by the way they look. they look totally factory is what im trying to say. now, the block appears as old and factory also. so the question is: are the blocks the same for 72 drum as with the 73 disc? if not how do i know the difference. right now the brakes feel like the rear sets down first before the front as the rear drops before the nose dips. hope this explanes it ok.