take a look at a nv5400. it is a 5 speed tranny that replaced the getrag. Available in 2wd and 4wd configrations. either 6.3, or 5.6 first gear ratios, and an overdrive of .73. 3.08 are already pretty good freeway gears. I'm not sure if a 350 could push your truck in overdrive as it would turn your 3.08 into 2.25 rear. 65mph at around 1750 rpms with 28 inch tall tires. might be hard to climb hills.
the first gear ratio is much lower than that of any 3 speed. the nv4500 is built to take the power and load.
