You can get a 6 speed in one of the base model 1500s but I would not the 8 is dramatically better with better towing & economy. I'd take the 3.21 8 over a 3.92 6 speed.
Ditto they should have the 8 speed behind both the HD gassers.
I have a 3.92 8 speed behind a tuned Ecodiesel. 345,000 towing TTs up to 9,100 pounds not the first problem with it. Change the fluid & filterpan every 100k.