I did the same thing, I traded in my 01 lightning for my 06 gt. If you're anything like me, you'll miss your truck. But the mustang is still fun to mod.
I was in the same predicament as you. Its hard to find a nice set of black 18inch rims for these cars but I came across the Foose Nitrous and absolutely fell in love with them. Mine are on order but should be in this week. When I get them on Ill post up some pix.
Not sure where you from but ill tell you around here NJ/PA/DE no one carries foose it took me almost 1 month just to find a place that could get them. Expect to pay anywhere from 1300 - 1500 for a set though