A 255 will work fine, if you are worried about it. I had those same exact wheels on my old '98 GT. Eagle bought them off of me and they have 255's on them right now. I never had a problem with that size on that wheel.Thats what i was planning on. maybe 245's.. thats what they came from ford.
Just to let you know, if you do go that low, you will need CC plates, lowering ball joints, a bumpsteer kit, and probably new shocks/struts too. Otherwise you will never have the correct front end geometry and your stock shocks/struts will wear out prematurely.it can
just kidding i havent tried but im hopefully going to drop it 2"