These engineering decisions need to be kept in context. A traveler on Route 66 in the 1940's would have found speed limits in the 35 to 50 range (and slower in towns). And I am told that the limits were reduced to 35 during WWII to save fuel (which was rationed).
Ike didn't authorize/fund the interstate system until '56. Most of the freeways here in Ohio date from the 1960s - with the 4xx bypasses and loops going in in 1980s.
These old cars were not designed with 70mph in mind - because there wasn't anyplace to go that fast.
I don't know when those ratio tags went out of use on the GM factory floor. I suspect there were others ways to ID different assemblies.
