biggest you can fit is a 285/35/19, but that is outside most tire manufacturers recommended tire width for a 9" wheel. Stick to a 275/40/19 if you want to maximize contact patch. If it will be strictly for looks and DD, a 285/35/19 is fine.
a 305 is a 12" wide tire....you can't stretch a tire sidewall in 1.5" on both sides and still expect it to hold a bead...
285 would be optimal, but I think you could run a 295 at the widest. My buddy has 275s on a 9.5 and the sidewall is sucked in just past flush with the lip. Like a 285 on a 9", 295 might be pushing the limits of a 9.5"