Kelsey Hayes Reman 4 piston calipers are still available at Rock Auto.
Check there for the new rotors too.
Along with the 15/16" bore master brake cylinders for easier pedal pressure and better brake control. Think that manual master is listed under 73 Dart manual discs.
Not necessarily so, anymore, these days, in the way of calipers, at Rock Auto.
They are pretty much "out of stock" all the time when you look, or only one side is ever available.
Those things are getting more difficult to find, all the time.
Look in the for sale section.