Since you are buying A Doge it doesn't really matter, but you don't need 3.73s on a Ford SRW. The gearset is designed for the 3.55 rear end. The dually gets 3.73s for the increased load capacity. But I recently towed some grades (the big ones were 7 miles at 6% and another one of 5 miles of 6%) with a friend driving a F450 (4.11) gears. We both held 50 MPH, with pedal still left, and towing similar weight RVs (15,500 lbs.) Both of us were running around 2,200 RPM. The only difference was I was in 3rd gear and he was in 4th.
Different tire sizes have little affect on the final drive ratio as the tire aspect ratio changes to keep the tire diameter withing 1/2 - 3/4" of one another. My tires on 20" rims are effectively the same height (and therefore the same circumference) as tires on 16", 17" and 18" rims. The shorter sidewall height of the larger tires does marginally affect the softness of the ride due to reduced flex area. But they look good.
