If you're anything like me I'd suggest that you not go an drive one. That ISB is a nice motor. Mine is the standard output 370hp/850tq. The HO in the 3500 is 400/1000. That will get the job done in a hurry.
So far best tank has been 13L/100km and the worst has been 14.1L hand calculated. That's cruising at 120km/h+ too. The Power Wagon burned 24L/100kms so expect Shell stocks to go down.