Sorry if this is too much info, but there are lots of variables in play when it comes to fitting wider tires on a Fox. The first thing I'll say is that, for performance driving (track & autocross), you want to run a square setup, not a staggered setup. Staggered wheels and tires create understeer in a car that already has understeer from the factory. If this is purely for looks (and you are willing to accept worse performance), many people decide to run a staggered setup.
My 1992 GT has 03-04 Cobra brakes with 96-04 spindles (I know the 94-95 spindles are preferred), and a 1998 Cobra rear end assembly. So, if you have 94-98 length rear axles, my setup is similar to yours. I'm running American Muscle FR500 18x9" 5-lug rims. These rims are listed as having a 30mm offset/6.20 BS. Most rims will have a number stamped on the back next to "ET". That number is the offset. My AM wheels have "30" stamped near the "ET". Take a minute to check your rims, you might find your offset.
My AM wheels sit flush with the fenders, I've never had to roll them; I have quad shocks in place, and I haven't had to clearance the inner fender area. I run Ford Racing C Springs, so my car isn't lowered very much. I have never had an issue with tires rubbing.
In the past, I used a 275/35/18 Nitto NT05 in the rear and a 255/35/18 NT05 up front. I currently run a 245/40/18 BFG Rival S 1.5 on all four corners. Tire choice makes a huge difference. My smaller BFGs are MUCH faster than the Nittos.
As for tire size... Tires listed at the same size can vary greatly. When choosing tires, look at the section width and the measuring rim width. You might find that a 265 from one brand is nearly the same size as a 275 from another brand when used on your rims. Deviating from the measuring rim width will also stretch or pinch your tires, slightly changing the width.
Not everything in these videos will pertain to your question, but there is info in them that should help inform your decision.
If you have any questions, please ask.