I have a 2008 911 Cabriolet with OEM 18" wheels. I have come into a set of 20" wheels which were on a 2006 911S. Tire Rack recommends 235/30 fronts and 305/25 rears. First, will these wheels/tires have enough clearance for my 2008? Second, would you recommend a 295/xx for the rear rather than the 305/25 (which are hard to find)? Thanks in advance for your recommendations/comments.