Search for posts describing the 3.55 ring and pinion gears conversion, which allows you to keep the original enclosed drive shaft.

I made this change in my 1954 Suburban, along with changing to LT215/85R16" radial tires. This resulted in about a 20% decease in RPM, allowing easy/quiet cruising at 60-65mph. MPG did not decrease at those speeds.

Make sure your brake system is good, and make sure all your front-end and suspension parts are good.