About to order a 2005 600 and need some help. I'm geting a 3500 with a auto 2wd 8ft bed. My 5th wheel is 12,000lbs dry. I guess about 14,000 loaded. Should I get 3. 73 or 4. 10's. If I go with 4. 10 what will be my rpm's at 70mph, and if I have to run out of overdrive what will be my rpm at 65...