a 245/45/17 tire fits a 17x8 inch rim a 225/45/17 tire fits a 17x7.5 inch rim, but it has a bit lower sideall, 4.5 mm perside~
It is FAR better to run the same size rim and tire all around so you can rotate your tires. I would buy ford brand mustang rims so you make sure the offest and backspacing is correct, the last thing you want is for them to stick WAY out or WAY in. www.gefracing.com
has good prices you can go by~ but you an order from many places. Ford rims are pretty darn cheap.... and go to www.tirerack.com
for tires, add in 40 bucks for shipping most tire shops cant TOUCH em.