What are you pulling with now, that the trucks you're looking at would be a downgrade capacity wise? Or is that a theoretical trailer weight?
Away from gcvw ratings and speaking to the trucks you're looking at in particular.
As said the deeper gears will only provide more "power" not more safety in an apples to apples comparison.
Shopping for old vehicles/machines, my theory is looking for the best condtion/lowest miles, NOT the exact preferred combo. If happen to find all 3 that is a bonus.
Speaking to the particular trucks you're looking at. My strong preference would be 6 speed manual regardless of gears. If you need more power, any cheap programmer will make up the gear ratio difference just on its economy setting for far less than gears.
If you need more power than that you'll have to make sure the rest of the truck is up to it (transmission or clutch), in which case then it's a debate, spend the money on an older rig or buy newer with mo power.
