There are numerous gear calculator on the Web. Figure out what rpm your truck would operate with what gear. Best to keep the the truck in its torque band while towing. Headers, exhaust and intake can help. Not sure if the results would be worth it. Will the comp cam maintain emissions? I would recommend looking into rear gears and maybe tranny shift kit upgrade. The tranny shift kit will make it shift a little abrupt. For gears I would start with 4.10. It sounds like you don't need alot. The other item with gears is the case break. You can change gearing and keep cost down if you avoid having to change out all internal components. 4.56 and 4.88 probably requires a complete internal swap. Does the truck have overdrive? Running down the road empty with tall gearing will wind out the engine.