My experience with 2-speed rears on these old trucks is that the second speed is deeper than the standard rear and intended for getting a heavy load moving. I've never seen one era correct that would allow you to go faster than your current high.
Something you might look at is one guy swapped the ends of the axle housing to the newer axle and then redrilled the hubs so the newer axle shafts which bolt up, allowing running the original brakes and wheels while having a later axle ratio. Do some searching and you should be able to find that thread here on the Stovebolt.
