Those wheels are $165 a piece and come originally on the 2010 GT500 convertible. They OE Tire Size is 255/45/18 front and 285/35/18 rear.
Having said that, 285 is absolutely OK, as FORD put those on from the factory. I have those as my winter set (although 255/45/18 all around)
285/35/18 is 657 mm in diameter
255/45/18 is 687 mm in diameter
285/40/18 is 685 mm in diameter
No spacers needed. If you want 285/40 all around you want to look closely up front for rubbing. I would go with the OE sizes, as those won't give you any issues.
2010 Mustang GT Premium Red Candy
3.55 Gears - 3dcarbon GT500 Wing - H&R OE Sport Springs, Koni FSD Shocks and Struts, GT500 LCA's, Edelbrock adj. Panhard bar, Brembo brakes front, 2013 GT500 rear brakes, FRPP X-pipe, 2013 GT500 exhaust, SVTPP Wheels with polished spokes, Polk Audio speakers, Redline Hood lifts, GT500 seat covers, 2013 HIDs, 2013 GT500 Rear, Roush TVS2300 SC Brenspeed 560R