Aftermarket brakes for a 70 Dart?

The four-piston K-Hs are arguably better than the (still very good) later model single piston units, which the factory went to due to cost considerations... The later units do have the benefit of being upgradable to 11.75" cop rotors, but bigger is also heavier- and your combination of driving doesn't necessitate it unless you're gonna be doing a bunch of corner carving.
The KHs are great brakes, you just need to swap rotors to get the big bolt pattern. The later brakes have better parts availability. Your call.
You can always go Wilwood, Baer, etc. but you'll be doing legwork to get those up and running.
Beware of "complete" kits- few actually are. And getting service parts can sometimes be a pain after a few years and the manufacturer has moved on to the next "latest and greatest", or discontinued your model due to low demand.