Seeking Advice -- Purchase of Older 3/4 Ton Tow Vehicle

I think that kw/00 right on, however I think you should stay with the 4:10 gears even in the 6sp trucks. Since you plan to mainly use the truck to pull a trailer. You also state that you will be doing your towing in CA and the AZ high desert. CA has the 55 mph towing limit and most 2-lane highway is also 55 mph, so you don't need 70 mph gearing.