Rather than use adapters, you should consider removing all 4 of your hubs and installing Mustang/Cobra hubs...since this is the only component you really need to change to accept Mustang wheels. Front hubs are typically $100/pair new, rear hubs need to be from a 99+ Cobra since those came with IRS (probably closer to $180/pair new). Then you can simply have a machine shop redrill all of your rotors for the Mustang pattern ($100) and reinstall everything with new hub nuts ($7 each). So as you can see...it's about a $400 operation for all brand new stuff. You can cut that price down if you scout some of the Mustang boards for used stuff. Almost forgot, you'll need mustang lug nuts now.
As for wheels. I have 18 X 8.5's up front on my SC with 5.75 backspace and 245/40's. They fit great. Nice and flush with the fender, plenty of room behind. My rims have a front space of 3.75 and IMO, this is perfect. Your rims would have 2.9 front space, so they will tuck in .85 inch further. Personally, that would be a little more than I would want, but they will fit just fine inside and out. If you are running the same wheel front/rear, I think it would look hot if you used some 1/2 inch spacers in the rear to give the car a nice even stance.