You realy won't gain anything swapping to the 2 speed. Those were generally used in 2 ton an larger, I have a few 1 1/2 tons with them, and are geared much lower than a 3/4 or 1 ton, even in the "high" gear. We're talking in the 6.50 range for high, usually in the 7s or 8s for low.
If you're loking for a more highway freindly final gear ration, there's 4 options, not counting bigger tires. 1, swap out the 3rd member for the late '60s/early '70s 4.10 ratio version; 2, swa the entire differential for a '70s or later diff with better gears; 3, overdrive transmission; or 4, add a Ranger Overdrive ($$$) or Gear Venders Overdrive (even more $$$$$$) to the existing trans. The easiest and cheapest would be the 3rd member or rear end swap. If you had a 1/2 ton, a 3 speed w/Borg-Warner overdrive or T-5 transmission would the way to go, but being a heavier truck, you need something with more stength. Choices are limited, and none of the HD overdrive offerings are a direct bolt on with the stock bellhousing. They can be adapted, though.
