Putting a 315-30-18 tire on a 9" rim is not safe. Maybe the Nitto is not as wide but I wouldn't do it. If you want 315's do yourself a favor and get some 18x10 or 18x10.5 wheels.
I ran 315 30 18's on 18x10.5 replicas. Unless they are the narrow nittos I wouldn't do it. If they are bfg or Goodyear get the wider wheels for the rear.