The KDW is an awesome tire. If you are going to the track I would get an extra set of rears and put the DRs on them. You never know whats going to happen on the street... especially with the heavier mustangs.
Kevin
Personally I think tires and rims are too big of an expense to mess around with and be unsure of fitment. The rims you chose will require spacers anyway so you have some play as opposed to having order a custom backspace like I just did but because you want to stuff as much meat in there as possible instead of playing it safe I would suggest you figure out exactly what you need. It would suck to find out you have too much tire and have to invest in larger spacers and have oit stick out past the fender. I would suggest instead you invest an extra $70 dollars in something like this or even fab up something of your own like I did to make sure the full tire width you want will clear everything up front.