The 275/40 is tall. Not only are you going to have the issue with the clearance at the front edge of the fender like what I addressed you may also have to deal with the tire rubbing the frame at full lock in the back. You can put a notch in the frame, I would do that before I tried limiting turning radius. Since you have different spindles, hubs and brakes than mine I do not know what your fitment situation is going to be. Your best starting point would be to find a stock 18" Mustang wheel as a starting point for fitment testing. If you have a friend with a Mustang that would be ideal. My front wheel is +35mm offset. For 18x9 that is 6.38" backspacing, for 18x9.5 the same offset would give you 6.63" backspacing. Please keep in mind that I have a different hub and brake setup, so it may be different for your car.
