Proportioning valve question for disc brake conversion

Chrysler spent millions of dollars to develop their braking systems so why not take advantage of that?
My 69 Barracuda used the complete manual disc/drum setup from a 75 Dart and it worked flawlessly. The plan on my 64 Belvedere is to do the same thing. And for anyone that might disagree a former automotive brake engineer and founder of one of the custom aftermarket brake systems companies agrees with this approach.