You'll need the Casting Number, Casting Date, and Assembly Data. The Casting Number will be on the rear (as installed) of the center section at about 10 O'Clock, and should be 3859140N for a '65. The Casting Date will also be on the rear of the center section at 8 O'Clock. The Assembly Data will be stamped on the front of the passenger side axle tube. The data will show two letters to ID the factory gear ratio, then four numbers for the month and day the axel was assembled, then the plant code (G, B or W). Stamped below the Assembly Data will be a D or a E if the rear end has positraction.To figure out what all those numbers/letters represent you can look on this site: http://www.myss396.com/ChevelleInfo/Differentials/1965_Differentials.html
Or post it here and someone will decipher it for you.
