My father's '92 GT has four cylinder springs on it and 245/45/17's. The tires do rub the against the inside of the fender in parking lots sometimes when making a quick turn(the front sway bar is removed so a quick turn in a parking lot = a bit of body roll that leads to the tires lightly kissing the inside of the fender). I'm not sure if the isolators were removed from his car or not.
Here's a picture of the car for ride height reference(don't mind the photo quality, photobucket compressed it horribly

)
http://i2.photobucket.com/albums/y33/mfewtrail/1992 Mustang GT/stangprix016b.jpg
Of course, as with most things, ride height will vary from car to car for a number of factors...so the only sure way is to install the springs and check for clearance. This can be a bit of a pain if you find out that you don't have enough clearance...but springs aren't that hard to swap out on these foxes anyways(at least in my opinion).