Just found something very interesting in the 1970 Service Manual for Plymouth vehicles. This could certainly add credibility to the thought that this Dana 60 is an over-the-counter purchase as it has 4.10 gears.

I quote:

“In some instances where a higher numerical ratio gear is installed, a new differential case will have to be purchased and installed due to the difference in ring gear mounting dimensions. The standard ratio gear set used with both the 440 and 426 engines will be 3.54 ratio.”