You will likely need to replace the entire rear axle to obtain a gear ratio that will give you your highway speeds, likely one off of a c50 or 60.
Another thing to consider is the reason the axles had such a low ratio in the first place; It was to deal with the lower power output of the motors of the era, which was why V8 engines on big bolts were such a big deal back then. You could swap out the rear axle from a motor home fairly easily, but then the anemia of the engine's acceleration will make itself apparent.
