1) Havn't done that yet. There are adapters that screw onto the output spindle of the speedo drive at the gearbox that can correct the speedo error. For 31" tires you need a 1:1.16 They are commonly used over here by the Suzuki guys I drive with to enable their vehicles to pass roadworthy and engineering inspections. I believe the threads on the speedo drives are pretty generic between makes, so it is a matter of selecting the right ratio only.

2) There are only a limited number of ratios available for the sporty rear end (and front), 4.777, 4.625, and 4.444 on very early models. (Axe, correct me if I am wrong)