I would leave the exhaust as is. Going with a bigger pipe diameter is not going to gain you any torque (probably lose a little). Always go with the best flowing muffler you can. Torque is not about backpressure, more about exhaust velocity. A poor flowing muffler affects scavenging even at low rpms, and will more likely just hurt power everywhere. Here's an explanation. https://www.carthrottle.com/post/wg3b6r9/ or see the sticky on in this section for even more info.
Best bet, is to get a very mild cam. You want one with higher lift (.520-.550), a touch more duration than stock (200-206 intake/208-214 exhaust), more advanced ground in, and tighter lobe separation (112 to 114). Something along the lines like this: http://www.briantooleyracing.com/truck-stage-i-cam.html This will be expensive as you'll need to delete the AFM, plus change valve springs, and get it tuned by someone knowledgeable in VVT. But you should feel a power increase across the board. And definitely keep the VVT, as that will help the torque side of things.
Other than that, it's increase displacement, Supercharger, higher stall converter (but not too high), and/or higher ratio rear gear.