What’s the best kit to put disc brakes on a 63 Dart

Scarebird is by far the least expensive. For cheapest price, buy just the adapter plates from them and source the calipers & rotors locally, ebay, or rock, usually GM or Toyota PN's. You will have to redrill the wheel lug holes in the rotor. I recall they provide a paper template. The other option is to change the whole spindle to a 1973+ one or even a later Volare/Aspen one. Many posts.