I did something similar to mount thosw big PBR 2 piston calipers onto a Jag front end. The discs are 330mm (13") x 28mm and come from a Holden Special Vehicles, Club Sport. This is a hot version of the current Pontiac GTO you guys get over there.
The bigger discs and PBR calipers are actually lighter than the original Jag XJ6 setup.
Cheers