I pull a 24' race trailer, under 10,000 lbs, and have a brand new rebuilt Standard Transmission (of Ft. Worth) NV5600 because the original nv5600 had no synchro in 6th (bought truck used with 180,000 miles)
Amsoil syncrhomesh fluid, and fast coolers and I see temps creep slowly to 230-238 degrees in 80 degree weather, engine temps 195 water. Standard Transmission say 30* over engine temp is usual.
I would prefer a 3. 73 rear vs. my 4. 1, as the sho vp44 and edge/juice and 4" exhaust are ample power for 3. 73. I believe 6th gear cannot take the high torque load as engine power will enable me to take most any hill in 6th.
Many views on nv 5600 capability to pull heavy loads in 6th. I am amazed at high temps, old trans with the special Dodge trans fluid had much lower temps, perhaps I should use it instead of Amsoil, but listers here prefer Amsoil, generally.