The 295 is the widest rated tire for a 10" rim, if you can get into a wider rim a 10.5 or an 11 here is what you can do for your Mach1
(based on 04Mach1)
18x10 -- 295.35.18
18x10.5-- 305.35.18
18x11 -- 315.30.18
You might have to modify to make these fit without rubbing.
17's are a little trickier. The widest should be a 305.35.17 but none of the manufactures we carry build this size so most choose to run the 315.35.17. Again, modification might be required to make fit.