That's why I chose XXRs, they had the exact backspace I needed plus I knew that a 9" wide wheel would be problematic without inboard springs or inner fender work, but 8.75" width would be perfect.
18 x 8.75 on all four, 6.25" backspace, +35 offset
255-40/18 rear
245-40/18 front
first gen Viper brake kit on drum brake spindles (narrower track), and a`69 b-body 8 3/4 (wider track) make it all work with no spacers or rubs.
Don't worry that 18s aren't everybody's cup of tea, you"ll never have to worry about tie rod interference like so many 17" mustang wheel users are having trouble with. -pauly