Gavyjoe

Hi, could anyone advice me on whats the best size of 18" slick I could run on my type r race car and best offset wheel to use ? It has the 6 speed running gear, hubs etc and has the arches rolled, currently was using a 24/64-18 Michelin slick on an 8.5 inch wheel, et 48 and a 3 mm spacer to clear coil overs on rear, which I thought would do the job, but have since been advised by Michelin that I need to be running a 9" wheel for that particular slick ! Any help/advice would be great as I really don't want to start cutting the shell to arch it ! Thanks