What you are really looking for is low rpm torque. Other than a twin screw S/C this is hard to make. One option mention above is bigger cu.in. Anther consideration is Thorley's Tri-Y headers...I believe they are still CARB certified, Level 5 Trans and PCM programmed for towing. The cam and tuning would be better optimized with the above. When it comes to mpg and towing, they are not compatible. The other helper would be a meth system.

The above on a stock 5.3 should give you between 35-60 ft-lb of additional torque below 3200 rpm. A S/C would give you another 120-150 ft-lbs of torque.

 

The meth system would only be a "helper" if he is using forced induction (Turbo, Supercharger). Typically they come into play as boost pressures rise, and the compressors efficiencies drop off creating more heat in the intake charge.

 

are you still runnin the stock 265's?
4.10's should help some,along with the for mentioned mods

 

a tune would help a bunch! lots of potential in that stock pcm, id go there first