Front or back? If it is the front i would go with 235s or 245 max. You don't want to decrease a fox's crappy turing radius any more than it already is.
If this is for the back, I have seen guys run 265s before, but I think a 255 is perfect.
Play with this calculator a little. I think you'll see 255 is about max. It's not the 265 on an 8" rim, it's also the series you'll run. I'll guess you'll be running a 35 or 40 series tire. 265 35 18 would be tough on an 8" rim. You might have trouble finding a shop that would put them on.
My Grand Prix GXP has 18x8's with a 255-45-R18 Bridgestone Potenza. Those are factory equipment. The fit is fine. For a Stang, I'd go with 245 or 255 max.