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.
