Good question. Kirkham used Wilwood 6 piston calipers with 12.2" discs for the no-expenses-spared billet car built for Larry Ellison, see page 17 here:
http://www.kirkhammotorsports.com/im...aoe/aoe_12.pdf
I'm also looking for a decent set of brakes that fit the 15" Halibrands at the moment and would be interested in what others did.
Simon