There are many threads here regarding this conversion.
@Ross Wooldridge is a pro on this subject and has taught me a thing or two about this setup!
The main issue with aftermarket kits is that they were not designed specifically for our powerful 4,500+ lb C-Bodies effectively. They’re a combo of Ford and GM parts that “fit” but again, aren’t meant for a car so big.
Your best bet is indeed a 1969-1973 C Body disc brake setup. You will need: Spindles, Calipers, Caliper Bracket, Rotors and the respective bearings depending on what year you’ll get. The 1973 spindles are unique, as they use a larger bearing that allows for use with rotors that Chrysler used for longer on their production cars (well into the 80s IIRC.) EVERYTHING will bolt up to your factory setup. Spindles and steering knuckle will fit like a glove. The hardest thing to find will likely be the 1965-68 Bendix disc brake booster. The factory drum brake booster will NOT provide enough assist to safely operate this braking system. Same goes for aftermarket boosters, they might work but it’s not a good idea to bet your life and others on a booster that is probably not strong enough to stop these cars.
There is a thread with a more in depth explanation. I referred to it when I myself converted my New Yorker to discs:
65 Fury disc brake conversion advice