Wider tires

Hey guys I am new to this whole forum thing, but I got a 1990 Mustang with a 5 lug and 4 wheel disk brakes on it. I bought some 17" bullitts from a guy and they didn't fit right away on the fronts, I had to put a 1/4" spacer on it to fit. but I wanted to put a wide tire on the back, but I have been told they widest I can go is like a 255 45 17, How can I reeally go wider?? more spacers??? Thanks guys