Have you looked at the American Racing 105MB wheels. I know they are only 17". I looked at 18" for my car, but I could not take the price of 18" tires.
In fact, the wife and I drove a new V8 Monte Carlo SS, but the cost of 18" tires turned me off.
http://www.sporttruckspecialty.com/wheels/vintage/torqthrust_m-painted.htm
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03 GT, STB, GM HD sway bars, 35% tint, GE893(37watt)fogs, Flowtech Terminators, 180 T-stat, ZZP DP & PCM, Ceramic PLOG & Crossover, 18" glasspack, 2007 17" Mustang Bullits 235/55/17 Pirelli PZero Nero, 12" R1 D\S rotors, gutted air box, GTP radiator