275 is the widest recommended tire size for a 9" rim.
You will start to see your handling suffer if you go outside what is suppose to fit on your rim.
I did quite a bit of research before going with 245/45/18 up front and 255/45/18 out back in a Nitto NT555. You wont have any problem with traction in this size if you go with the Nittos. These are the stickiest street tires I have ever owned, almost frustratingly so.
On my bullitt I have 18x9's and breaking traction is hard in a straight line even with the 5 speed and gears w/TCS off. Handling is right where I want it though.
97 Thunderbird LX 4.6L
PI Intake Manifold, Comp XE268's, 180 degree T-stat, Brother's Performance U/D pulleys, K&N filter w custom CAI, 75mm TB and Trick Flow plenum, Borla mufflers w Magnaflow tips, Magnaflow straight cats, Summit 8mm wires, Eibach Pro Kit Springs, Koni Adjustable Front Shocks, Bilstein Cobra Rear Shocks, Cobra R replica rims(17x9)front (17x10.5)rear