When we wanted disc brakes for the dirt track race cars I built, we would pick a disc and caliper setup to match the wheel bolt pattern of the hub, clamp the caliper onto the disc with air pressure, and then fabricate caliper mounts to marry it to the spindle, using the same flange where the backing plate was bolted. Make the prototype brackets from plywood, then when they fit the way you want, duplicate them in steel. "Kits" are for people who can't or won't do their own fabrication.
Jerry
