I'm running a 6.2 with 4.30 gears pulling a trailer rated at 15600, that's also what the truck is rated to pull. I have no problems towing it and it never has to run at red line unless I want it to. Most interstate or flat two lanes it just runs along just under or at 2K rpm with the cruise set. Remember one thing lower gears does more than raise the rpm it changes the torque multiplication so matter what gear you are in you will have more HP and torque at the point were the rubber meets the road. You can have a 1000 HP but if the final gearing is wrong you have defeated all the power the motor can generate, the new 6 speed with 2 OD gears makes up for the lower gears when at hwy speeds.
Denny