Rustednuts is right 6.17's wont help that much. My truck has 6.17 with 38 inch tires, and a factory governed max speed of 49mph, however I never even run it that fast. I keep her between 40-45 mph and drive other drivers nuts. I don't like runnign the engine that fast.
However I'm going to try something... just last week I pulled a 4 speed tranny out of a really bad shape 1950's international 2 ton (picked it up for 10 bucks). Gear ratio's are really similar to my trucks tranny. (4th is 1 to 1 and 3rd is about 1.7 to 1.
Anyway I'm going to install it backwards between the original tyranny and the diff and run a linkage system up to the cab through the starter button hole (my engine is a 1959 and has an electric actuator that eliminated the starter button). The linkage will only shift from neutral to 3rd and 4th. This will give me normal speed in 4th gear and in 3rd it will give about a .59 step up. Making my truck cruse at 55 at a bit more than 1900rpm. It's a bit more of a step up than I was looking for, but it's better than buying a $3000 overdrive spliter.
