My opinion...
Congratulations on your purchase! I just got my set-up, only in Anthracite, and have nothing but good things to say about Bud at Wheel Replicas and the product.
You do NOT need spacers to fit 275/40 and 315/35 tires on your Mustang. However, the "technicians" at Discount Tire, Co. would not mount my rear tires because they thought I did. They essentially said, "Sir, these back tires clearly won't fit without spacers. It's time for us to close, you're out of luck."
Well, needless to say, the car was still on the lift, completely in the air. This was important because the chasis and axle sag a bit, which extends the horizontal rear axle damper in the back. This was where the clearance issue was. I went home, and jacked the car on each side individually and put the tires on myself. As well, I shook and pushed the car as much as I could to simulate bumps and the car shaking while a friend under the car reported no contact.
In my opinion, I do not think the front or rear wheels stick out hardly at all. Even so, I think they look a bit better when they do. I have a picture of my friend's Camaro SS with A.R.E. 200S Wheels, 17x9 and 17x11-inch, that stick out a lot if you'd like to compare with the pictures on my site of my car. Let me know...
- Brian