You can get an extra set of wheels that will look exactly like what comes on your CSX 4000 from Vintage Wheels (in 15" or 17").
http://www.vintagewheelsus.com/index.html
I got an extra set of 15" rims from them, and I was very pleased. Vintage should be able to tell you the exact rim and tire sizes that will work for your car.
Regarding Shelby, I would suggest calling Gary Patterson directly (call main Shelby number and ask for him), instead of dealing with other folks there.