255's are too narrow for a 9.5" wide wheel and the rims will stick out noticeably further than the tires. 265's would be better, but I believe 275's should be minimum and 285's or 295's likely your best fit.
A 295/40/18 or 265/45/18 is the same height as the factory 235/50/18 tire, so maybe put 265/45/18's up front and 295/40/18's out back.
For really good dry and wet traction for all season high performance driving, I recommend the Pirelli PZero Nero M+S. I bought them for my 03 Mach 1 (275/40/17's) and they got better traction in the rain than the factory tires on dry pavement and snow traction was OK (although, not great). Very affordable tire, also, and bigger sizes are readily available. These should not be confused with the standard Pirelli PZero that comes standard on Mustang GT's with the 17's - those are crappy tires. The PZero Nero M+S is a far superior tire.
That is the same wheel size on the Shelby GT500. Here is an excert from an Edmunds review of the Shelby regarding the wheel/tire size.
Mr. Shelby says, "Make it so"
A specific upgrade directed by Carroll Shelby himself had to do with the GT500's tire size. After driving a prototype, the Texas chicken farmer insisted on larger rolling stock to better manage the car's power and handling capabilities. One look at the car's 18-by-9.5-inch aluminum wheels and Goodyear F1 rubber suggests he got his way. The 255/40 front tires contrast against the larger 285/40 rear tires to give the GT500 both functional and visual appeal.
Those tires sizes mean the front will be 26" and the back will be 27" tall.
I'm pretty sure they're 255/45 18 in front, so they have the same diameter. I'm running the 255s all the way around on 18x9.5 on my '05 GT. The tread is about the same size as the wheel width, so some may think they tires look a bit small for the wheel. I like it, and it gives great responsiveness. You could go wider, but would probably give up a small amount on the responsiveness - you're call on whether that matters to you or not. I'm running the BF Goodrich KDW2s and am very happy with them. Much better than the stock PZeros that were on the car when I bought it, but they were fairly worn so it wasn't really a fair comparison.
I would say 275s because I have had 275/35s on 9" rims in the past and they sit just right. I have a 275 on a 10.5" rim on the back of mine and they look too small. I'd say a 275 would be perfect for the 9.5" rim. Bear in mind if the tyre is a stretch, the rim sticks out and will be the first thing to rip itself off on a kerb. Proper fitting tyres will give your rim a little protection as the sidewall should hit the kerb fisrt - not that you should be hitting kerbs at all but it happens!