245/45/17 is the OEM size on 17x8 factory rims. 255/40 is way too short, 25" tire probably looks ridiculous up front especially if you are not lowered yet. A 245 will fit a 9" wide rim too but just barely. Again, depends on the brand. A BFG Comp 2 Sport will run a little wide so if you are worried about fit, get one of those.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.