Sounds like your going to be hauling heavy and often. If that's the case, and I hate to say it... buy a duramax. Then you can have mutiple tunes so you can haul your cattle trailer around, then unhooking and turn a knob and unleash the beast! You said your willing to sacrifice HP for tq. How about 900 ftlbs with a stock Turbo and drive train? I think you will spend alot of time worried trying to tow heavy loads on hot days with the Turbo gasser. I'm no F I expert tho either.
99 silverado 2500 4x4 ec lb, lq4 with 243 heads, pace setter long tubes, true duals with x pipe and bullet mufflers, comp 222 224 112 cam, 8.1 truck injectors, 4l80e with 2800 stall, 373 gears, 285 75 16 tires, efi live tune in progress, 350rwhp 360tq, 11.8 average mpg mixed driving