That may be a little much for offset, with a 9 inch rim, the new stangs are going +35mm and with a 10 inch its +45. If they do fit, the rim and tire will be alot farther set into the wheel well than most others. before you get the tires mounted, i would pre-fit the rim and see if you have clearance..
when you test fit the rim make sure you have the suspension fully loaded to simulate driving conditions tho and check the turning clearance for wheel rubbing the caliper, control arm, or frame (slightly more accuret)