Best bolt on mod is premium gas, a clean air filter and good plugs. The stock 5.9 should handle 7000 lb OK if the truck chassis is up to it. I pulled 5500 # through the hills of Missouri with a 5.9 Durango with 3.55's - no sweat.

On hills, especially in hot weather, it seems that the knock sensor kicked in on mine and I lost torque. Premium gas prevented it. Probably why it was suggested for towing in the owners manual.

Beyond that, headers and a low restriction exhaust would help some. I would stay away from any of the chips or programmers... not recommended for towing.