I don't know if I'd go that wide of a tire out back on a 9.5" wheel. It could be done, but the sidewall may buldge a bit. In the front, I'd probably go 255 or 265 max, so you're not risking any rubbing. Too wide, and you may be forced to run some rack limiters.
i run 275's on my 9" rims(front with no rubbing problems) and with a 9.5" i would run probably a 285/295 for the rear. You could even run 275's on the rear if you want to be able to rotate your tires...