How much room do you have in the front of the rear tires? Are you at about the limit height wise for the rear tires not making contact with the front edge of the 1/4 panel?
I have about an inch and a half to the front with a 26 inch tall tire. You could probably get a 27 inch tall tire to fit, but I think you would have clearance issues with the quarter where is goes in unless you jack the rear end up.