I was on jegs and found this wheel simulator it has the G body monte but of course not the Grand Prix..... :? I don't know about any other g bodys though. You can pretty much customize the car to your liking. Its pretty cool to see what the wheels would look like before you buy them. 😀 Here is the link if your interested...
http://www.jegs.com/c/Wheels-Tires/10573/10002/-1
http://www.jegs.com/c/Wheels-Tires/10573/10002/-1