I'm not sure Chevrolet ever made "electric shift" for their rears. All of the 2-speed electric rears I've ever seen installed in Chevrolets were made by Eaton and they don't interchange with GM vacuum rears.
Wire brush the front of the center section of the rear to find the stamped numbers...the ratio should be there. It would be a shame to go to all of the trouble of replacing the rear only to find out you didn't gain anything. The guy you bought the rear from may have kept his foot on the floor or been going down hill to maintain 65 mph.
Mike B
