to 1/2 like stated above. The 12mm are really too small on the front anyway.
I actually changed mine to 14mm on the C5/C6 hub. Used a Porsche racing stud from MSI Racing. They were about $10 or $11 a piece. Yes you will need to drill out the hole a small amount, but that does not solve your problem of 2 different size lugs.
These may be too long but you get the idea on a 1/2 stud
http://www.jegs.com/p/Moroso/Moroso-...48727/10002/-1