Sorry didn't read this thread in its entirety , but the Title made me remember,
My buddy Raphael just tore apart a Legacy 91' and has the front brakes available, and i think between Ansel and I we have some rear calipers (older impreza) .. Actually i have an other set of front impreza calipers (made for the 9.5" discs though ...
If any of that helps you out LMK , if not i'll keep em for rally spares.