i'd say the easiest way to tell would be to have a good feel around your arches to see what kind of clearance you've got at the moment, then judge what you'd have with wider rims. either that or get a spacer? or just get your rims done and get the arches sorted, using the whell as a guide. better than guess work. if you really want 8" rims you'll have to do the work anyway so you may as well just go for it!!!