The code on the fender tag of a RO/WO calls for a special order axle. I don't think they swapped them 3.54's it would be a waste of time. I think they just built them for the car with the 4.88 truck gear set in a car housing. Just like the rest of the car, it was built to spec, not a converted production car.

A unique differential is needed. Had to have the centering buttons inside for car axles, but be able to use on 4.88 gears, a carrier for the 3.54 gears would not work. This differential needed to be assembled ahead of time. This could be done with a truck sure grip differential housing with passenger internals.

Only problem with this idea is there a 1967 4.88 B-body Dana on any BOM list?

Would be nice to check a known original 4 speed RO/WO Dane rear and get the numbers.