I'm making the brackets to put the Wilwood calipers on the Jag vented rotors. I think the rotors are readily available XJ6 and will fit your set up. We'll compare what you have with what I have here. I have made rotor hats as well as caliper brackets to get from here to there. Check my parts page. This set up uses an adapter from Jag hub to Wilwood rotor
http://www.classicjaguar.com/eng_brake.htm