One method would be a center section from a 60-72 truck with the GM/Eaton axle. They typically have the 4.11 gear, you may luck out and find a 3.90.
A little clarification; The third member you would want is from a '68-'72 C20, generally with a 350 and automatic transmission. All others will be either a 4.56 or 5.13. The 3.90 mentioned is not an oem gear, they were a low production aftermarket run that seem to be impossible to find. The company that had them made seems to no longer be in business, though their website is still up.
I've been told by a knowledgeable person that there was indeed a factory 3.90 gearset, used only in some big block 3/4-ton pickups in the 1969-71 era. He's the only person I've ever talked to who has seen one.
Some manual and leaf spring applications would also have the 4.10, but the auto/coil combo is the most common.
No need to count gear teeth, either use the method outline above this post or look for the markings on the edge of the ring gear.
