If you're dead set on gas, definitely get the 4:30s, (You'll have to order the truck to get them), and you may want to contact 5-star for a tune as well. The 6.2 will do it, and some owners tow that much, but towing over 10K I'd probably recommend a diesel. kinda depends on the terrain, and distance you pull the 5er. Going 350 in 2wd, the payload will be no problem.

I get just under 16 on the highway in mine. Combined average so far is 13.8, and towing my 7500lb trailer I get 8-10 depending on the wind, hills etc. Good luck.