Aftermarket brakes for a 70 Dart?

Manual in all. First, KH hubs and calipers are getting harder to come by. They work real well when they do. Wilwood (Conversion kit $$$) to me stop about the same. Some may argue that. The stock 73-74 (More years and models if you look into it.) Easy to maintain and get parts for. I thought they stopped just as good. I put a 100k on them and only had to change pads when needed. Never needed a caliper or a rotor (Not even turned). As said JMO.
I'll add one thing- if he uses the Mustang rotors to convert to the large bolt pattern on the K-H brakes, rotor and hub availability is no longer a factor; they are commonly available. Plus, you rid yourself of the Mopar's 2 piece hub and rotor and get a 1 piece unicast hub and rotor- no more pressing wheel studs in/out to change rotors and dealing with swedging.