I might suggest a tuned port injected 350 out of a late-80's Corvette or F-body. The "TPI" makes extra midrange torque due to its design, and is the best truck engine GM never put in a truck.
Down low, a TPI 350 will pull harder than an LT1 or an LS1. A properly built TPI engine will feel stronger than either until at least 4000 rpm, and that's where a truck needs it.
