98 ford mustang gt tire size?
Dave is correct that the 235/55ZR17's would be slightly taller than the 245/45ZR17's, and that it would effectively make your gear ratio taller (numerically lower) and result in your speedometer reading a couple mph lower than your actual speed.
That being said, I imagine that the tires you are considering are quite possibly stock take-off tires from a 2005+ Mustang w/ 17" wheels. In the 2005+ Mustangs, the standard gear ratio was changed from 3.27 (for the 99-04 cars w/ the same 245/45ZR17 tires you have) to 3.31 in order to account for this slight change. However, it is certainly not unprecedented for 94-04 Mustangs to use the wheels/tires from an '05+ Mustang, so I would say that using them on your wheels would be fine.
The only concern I would have is that the new tires will be visually taller because of the higher profile. If that doesn't bother you, then go for it.