you use the s13 rotor and caliper. They are 250mm discs and a single pot caliper. They would be perfectly suited to your r31 rear brakes and I would go as far as saying there would be a few others on the site running that exact combo. I believe Kev Rowley is the man to contact about the caliper bracket. Someone on the site will be able to chime in with his contact details.
I reckon that would be the simplest most cost effective option for you with the bits you already have. Plus bolt on mods make life so much easier. S13 rotors and calipers can be brought for nothing since all the silvia boys swap them off for the big nissan four pots

edit: I think they may require the hubs to be turned down a couple of mm so the rotor will fit over the hub. Its all pretty minimal stuff that any engineering shop can do for cheap/beer.
Have a quick search on the site its been covered quite a bit.