I have a 01 cobra with some fr500(10 1/2*w) in the rear. I wondering what is the widest size i can go with and do i need those irs bolts? Currently i'm running with 285/35/18, but this shop in LA is selling some hankook tires sized 315/30/18 for $80 per.....Would 315's rubb or is the 30 series to low in profile for dd quality??????????????