I would try to find a truck with 4.10 gears and one with the factory tow package (which improves on stock cooling for the engine and the transmission). There is also a point where the 5-speed and 6-speed transmissions emerged and these add one or two overdrive gears and can help with fuel economy.
Whatever you buy take the time to get it into a shop that does front end work and have them check out the ball joints and tie-rods. Not an expensive repair if it is done before you flip the truck on the highway when something comes apart.