If you have to ask that I highly recommend you just go with a 315 or take it to a body shop. You'll need to go SRA, shorten up the axles to pull the wheels in further towards center, flare the fenders etc...
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 ****.