Coming from the 6.4 Powerstroke to the 6.0 Gasser its going to be a dramatic difference. Personally if I was purchasing brand new both the Ford 6.2 and the new Ram 6.4 are putting down much better numbers. I have nothing against the 6.0, but it will be a big difference from that 6.4 with stock 650 torque. If you are dead set on the GM / Chevy 6.0 then go with the 4.10 gearing.
Brother in law still tows a 28' 7,000lb travel trailer and typically slows to 45mph on most of the steeper grades up here. When I had my TT with the 6.4 the same hills I would just boost and continue on @ 60mph. The 6.0 Vortec is proven, and as stated that's why they have not chosen to replace it. If your fine going slower I have been told over and over the 6.0 is a 250,000 mile motor without any major repairs.