Are you planning on running 17" wheels? Classic Revival make a kit to fit 330x32mm rotors and Willwood 4 piston Calipers to fit Jag XJ6 S1,2&3.
If you are running 15" wheels you can improve the stock XJ6 setup. I've got a Series 3 front end and I rebuilt the 4 piston calipers with new seals and stainless pistons. I also cross drilled and slotted a set of new rotors and put some softer pads in. I'm running a Tilton balance bar setup on the pedal as well. This fits inside a 15" wheel and would probably perform much the same as the equivalent sized Willwood setup for less $$$.
I'm planning to machine up some brackets and hats in to adapt 330x32mm ex V8 Super car/touring car rotors and probably AP 4 or 6 piston callipers. It'll have to wait till next year when the car is on the road and I've saved up some cash.
Cheers
