I have a set of rear callipers from an FSO/X19 I think, with addapor plates that fit 13" wheels. Unfortunately not fitted at present. I am thinking of just upgrading the wheel cylinders with ones off the front, these have a larger piston diameter, this is a suggestion Gadge came up with and it should provide greater breaking at the rear without the potential of locking up prematurely. It also removes the need for adapters, modified cables and pressure valves.
Ralph