Several people have done that on the Mach1's.
Up front you have to grind some off the caliper and maybe the banjo bolt depending on the wheel brand and will have to use a 1/2"-1" spacer to clear even then. Some use a 15x4 front with a 1-3/4" backspacing and others have 15x3-1/2 using a 2" backspacing and with a little grinding are having no problems.
Out back a 15x8 with 5" backspacing will work with certain wheels and have no contact with the stock caliper, but if a 15x8 with 5.5" backspacing is used they will require a small amount of grinding. Some people have even changed out the rear brakes to the GT's set-up to rid themselves of any fitment problems.
I'm not sure of which brands do what so I'd suggest findiing a wheel and tire shop that has a good stock and try out a few and see what you can come up with.