GM shows no listing for a 3.90 gear for the Eaton rear. The only one I know of was the early '55 sets that were made for the Napco 1/2 tons, which used the 3/4 ton left hand axles for the front, as the did not have a 1/2 ton version available. GM also has listed the later C20 axle as both a "4.10" and a "4.11", depending on what year specs you're looking at. However, it also goes further and clarifies that the 4.10 is the Eaton, and the 4.11 is a Spicer. That information is from the GM Heritage Site.
