Does anyone know of what calipers plus mounting brakets fit on 312mm discs?

you need to find 996 rears, boxter fronts or boxster s rears for 312 disks not the boxster rears as they will not accomodate the width of the 312 disks. There are some good topics on Uk-mkivs.net and there are a few seller on ebay doing brackets but if you search you might find Cad drawings that you can get machined in a local fabricators.