I'm assuming these are 17x8s correct? You can effectively get away with a 27" tall tire out back if you are going for drag strip. I had a set of 26.6" tall tires in my last car that was lowered and they fit fine.
Just get on tirerack and see what the actual overall tire diameter is going to be of whatever you choose.
That said, a 255/45/17 is a very nice tire to go with at 26.1" and was an OEM size for that rim (on 94/95 Cobras). Stick it with some 4.10s and your .68 final overdrive in that Cobra and you should be able to do a decent clip on the highway without too many RPMs.
Hit up tirerack like I said and then google "RPM calculator" and you can plug in all your info and see about what your RPMs will be.
edit: Also be aware that too much size difference between the front and rear as far as diameter is going to cause your ABS controller to kick off and stop working and you'll have that light on the dash.
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 ****.