1968 C20 transmission swap advice

[Ol' Chevy]

Howdy,
I have a 1968 C20 4x4 with a 4-speed transmission. The diff's are geared at 4.56. It really screams going down the highway, and the gas mileage is terrible. I was considering re-gearing to 4.10, thinking that may at least quiet her down on the highway. I am now wondering if I can achieve the same result by swapping out the 4 speed transmission for a 5 speed. Has anyone done this? Did you gain fuel mileage? Lose power? Any input will be helpful. Thanks.