HI Masa, They should fit the 2007 GT500 perfect since they are realy made from BBS for the Mustang and their variants. As far as fitment I do not know yet as I am still trying to find the right tire to put on the rim. I am not sure if I am going to do a 275, 285, 295 or 305 tire yet? Once I order the tires I would be happy to go int more detail.
Not sure how much I was able to help but I will be back soon once I find out.