Best way to increase torque for towing?

What do people think is the best way to increase torque for towing? I have a 5.3 liter 2001 Suburban towing a 6000# trailer. I want more torque for a trip to the Rocky Mountains. Tuner? Cold Air intake? Exhaust? All of the above? Will the transmission take the extra torque? Do I really need to buy premium fuel if I tune it?
Or do I have to simply break down and buy a Duramax Truck?

Headers, cat back exhaust and power programmer III