Tire size

255/35/20 is hella small tire for rear of car..he could run a 275/35/20 and it will windo up being about same cross section as a 275/55/15 maybe about .25" wider...I have seen 28x10.50 slicks fit on stock mustang with correct backspacing so a 20"x10" rim with a 275 and lower profile shoudl work fine...255/35 will probably cost more also sense they arent as common in a 20"