The dually from the factory HAS the offset hub on it. so how is adding this to a non factory dually to be an issue?
With this in mind, a set of 225-70-19.5's would net you 9000 lbs of tire capacity, 245-70-19.5 upwards of 11K lbs of tires per axel. The 225's would be no diam difference if you already have 265's on the truck. 245;s would be about 1-1.5" more in diam, reducing your overall axel ratio to about a 3.55. Not my first choice, as these rigs are geared a bit too tall for steeper hill ie 20+% grades for starting power.
Marty