Hi Spec 285s go in behind Willy 15s. Im pretty sure they wouldnt fit behind valver 6.5" wide rims though. Or you could go for the DIY approach like Ben suggests as buying all the bits separate is in this case, actually a lot cheaper than buying a kit. Some people like messing about with brakes and will do this sort of thing to save a few quid, others would rather get a kit thats designed for the job. Totally up to you and how much money you want to spend really.