275/35/18 will be a good rear size for that rim but some brands run wider or narrower than others. For instance, BFGs generally run wide so a 275 might actually be TOO big for a 9" wide rim with them. Nittos run narrow, on the other hand, so you might need a 295 Nitto but it'll be the same actual width as the BFG. You can check actual section widths on tirerack usually.
Just remember wider is not always better, you have too wide a tire on a rim and it's going to actually DECREASE performance.
For the fronts, 17x8 is the OEM rim size so OEM 245/45/17s will get you to the 25.7" stock height.
You can also go with a taller tire out back like 275/40/18 for 26.6" if you wanted to fill up the wheel well a little more or decrease highway RPM slightly. You will need to recalibrate the speedometer or it'll be off 5mph or so.
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 ****.