255/4017's suck on SN95's, & they suck on 17x8's. They're too short, so the car looks like a lifted 4x4. They physically fit on the 8" wheel, but they're a pretty tight stuff on there, & they track & feel poor. The only time I like a 255/4017 on a Mustang is on a 9" wheel on a Fox body.
I know it's boring, but the OE 245/45 is the best fit on the 8" wheel. Or you could do 255/45, that was stock on the 94-95 Cobras, they're a little wider & taller than the stock 96-01 245/45's.