wanting a 2-speed is plenty of reason to switch!
I think it also depends on year. Looking again, the 55.2-59 shows 2 ratios, 5.83/7.95 and 6.40/8.72 earlier probably uses a different rear.
Yes, the earlier ones did use lower ratios because of the smaller, less powerful engines. The 261 was available by 1955 so the final drive ratios got a little steeper.
Still waiting on someone to provide some ID data on these axles. I haven't been able to find anything definitive yet.
