Lovely looking car. What are you intending to use it for ? For fast road /track use the current wheels/tyres should be spot on. The much wider rear tyres you are talking about would be unusual for a road car and unless you have a 650bhp motor and having trouble getting rear grip probably not needed. Maybe you could get some information from some of the guys that used to race the RS500s in Australia to see what size wheels and tyres they ran and how they modified the bodywork to get them to fit?