I spent a lot of time looking for a K-H disc setup and finally quit due to high prices and poor availability. If you can really get rotors at $38, jump on them. The $21 might never been real. A year ago, Carquest listed the rotors at $60 but showed them as closing out at the end of the month. The Centric ones in the PartTrain link above don't look at all like K-H ones, so be skeptical, though '71 Darts should be the same K-H brakes.
In my case, I decided to rebuild the drum brakes and "roll-my-own" disk setup in the future, though it is trickier than it first appears. Several companies sell conversions, but none are totally elegant and all fairly expensive. Several make you switch to later upper control arms. Scarebird makes you re-drill rotor stud holes and grind caliper housings (depending on your wheels). I may post my solution when done someday. Perusing Pick N' Pull, it looks like '95 Camry calipers w/ 96 Voyager rotors (5x100 mm, 10"D) could work, with a custom mounting plate. Some issues are: strut attachment (bracket bolts to front), rotor bolt pattern, rotor axial position ("tophat" depth), hub register hole, hub clearance ("tophat" ID).
