Prop valve doesn't change, though you'll want to put smaller wheel cylinders in the 10" rear drum brakes to prevent the grossly premature rear lockup the disc brake A-bodies tend to have -- it's aggravated with power brakes. See
here (and threads linked from there) for details.
Booster and brackets don't have to be from a '70, you can use the easier-to-find and more compact '73-'75 item, which also gives better, less mushy pedal feel. Find a used one and send it to Booster Dewey in Portland, OR for a
good rebuild ("remanufactured" items are junk).
You'll need to make new master-to-prop-valve lines because the master cylinder will move upward and forward. Don't forget to put a couple of loops in each line, as the factory did, to allow flex without metal fatigue. Get a
new name-brand (Bendix, Raybestos, United) master cylinder for 1974 Dart with power disc brakes.
And you'll need a suitable vacuum tap in your intake manifold.