from what i have read, a 305/70 "may" rub on the control arm at full lock of the steering. same with a 315/75 as they are about the same width. I have had no trouble running 285/75/16 (same height as a 305/70, just not as wide). Some have run 305's and 315's and claim no rubbing. Either way, the rub isn't bad and lots of guys run up to 315's on stock trucks.
-Jason