Rotors for Kelsey Hayes 4 Spot Brakes

Since you already would have to replace the hub/rotor assembly, consider 'upgrading' to the later (A-body) single piston sliding caliper version. This would give you the all-important spindle and caliper (to use 'as-is' or exchange). Your stock upper control arm has smaller ball joint but can also be used with small insert. Still lots of donor cars out there. What is your plan to change rear axle bolt pattern?