Those 245s will be totally fine, the difference in overall height and tread width is minimal. Might actually look a little better with more flush sidewall honestly.
Also on tirerack.com you can look up any tire by size, click on a random brand and then click the "specs" tab. Will let you see the exact differences in height/width etc... Being Pirellis you could probably find your exact tire on tirerack.
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 ****.