Also going to want a 40 sidewall if you go with a 255 front. Hell, on my 98 I had a 235/45 up front and 275/40s out back. 26.3" tall up front and 26.6" tall out back. Filled up the wheel wells better and smoother ride too.
Remember, the sidewall measurement isn't static, it is a percentage. You need to look at overall tire diameter too and you want them to be pretty damn close. This is why you have to be so careful of the pre-packed wheel/tire combos out there. I've seen some pretty horrific tire size selections and one in particular on AM that just makes me #facepalm every time.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.