If you have stock wheels, then the stock 245/45/17 size is probably the most ideal to run.
If you want a little bit more meat, then 255/45/17 would be good and is what Ford put on the 94-95 Cobra with 17x8 wheels. It will put your speedo off a tad as it's a 26.0" OD tire vs the stock 25.7".
You really shouldn't put a 275/40/18 on an 8" wide wheel, although it's been done. You'll have some excessive sidewall bulge which looks great but can be detrimental to handling performance.