I would check with the tire manufacturer on the 265 issue regardless of what the shop says. I would rather believe the company responsible for getting sued.
No court in the world would accept a lawsuit from someone voluntarily putting the wrong tires on their car!
You could fit a 295 on the 10" rim with no probs. Nitto has a nice tire fitment guide on their website for their tires. Chart that lists all the tire sizes, range of rim widths they'll fit and the rec'd rim width.
I just put some 275's on my car but I went and got me some pretty 17x9 rims to mount the shoes on. I would only do 245/45 17's or 275/40 17's because the rotation rate is the same. You could always re program your speedo but I'm not sure how tall the 255/40's are.
I run 255/40/17 up front and 275 out in the back, the rears look a litte weird because it bows out a little. If you want tohe car to hook up good buy 17x9 rims for the rear and run 275. Mustangtuning.com sells 17x9 bullitt (notice that there are two ts) wheels that match stock color.
I just put a set of used 275/40s on the front of my car, they look GREAT! I already have 275/40 sumitomo htr zii's on the rear, and these other tires seem to be a wee bit narrower...it fits the stock 17x8 wheel so much better.
Plus, my cars lowered 2" + a bullitt swaybar and theres no rubbing on anything.
Sorry for the big blurry pic :o