5.13s is what I would go with assuming they make that ratio for the front differential. I forget what options those have available. I had 5.13s in my Van with the old 350 and it only had 31.5" tall tires on it at the time. 5.13 put you at 2,400 rpm @ 75 mph with a 4L60E. That is 100 rpm more than my Express currently turns with a 383, 4L85E, 3.73 gear and 30.5" tall LT245/75R16 tire. GM has used that combination on loads of 5.7L and 6.0L powered trucks and vans over the years and they all cruise down the highway nicely. IIRC a stock 4x4 of that era also had a 245/75R16 and a 3.73 gear from GM. That combination with the 4L60E is about 2,200 @ 75 mph. 5.13s will get alot of the performance back lost with the taller, heavier tires. With the lift you will be pushing alot more air and the gearing needs to be adjusted to compensate. My Express would not even blink an eye running 85-90 mph @ 2,700-2,800 rpm for hours on end on a long trip back when it had the 350/4L85E and 3.73s before I put the 5.13s into it. Before the gears it would still get 17-18 mpg running 85-90. After the 5.13s went in for towing, I held it down about 80 at the most. It would still get 17 mpg at 80 mph with 5.13s, I tested the difference prior to the gear swap by driving at 80 mph in 3rd gear for over 300 miles, made nearly zero difference in fuel consumption and almost no difference in the engine noise.