Couple questions before I do my brakes

Very old post I know. maybe someone can clear up a few things for me.
I am doing the front disc conversion (rear drums 11.5 inch) using spindles, callipers, brackets, from a 76 dart.
I was of the mindset that I could use the e body 2 piece proportioning/metering valve for the most responsive braking. But then I had read somewhere that you cannot use the 2 piece (combination type) valve on a body cars. Am I missing something here? Why would that be, maybe because the combination type valve is overly/under sensitive for a lighter a body car? IDK, it does not make sense to me.