As to question number three the casting numbers on the case will identify if it is a Posi or open rear end. For a twelve bolt Posi-traction rear end with two series gears the casting numbers are ED 32088, for three series gears the numbers are 30140PM1, and for four series gears the numbers are EDB 30174. For a ten bolt housing the Posi-traction numbers are ED32118 for two series gears and EDB 30116 for three series gears (there isn't a four series posi carrier for a ten bolt).
My Colvin "Chevrolet 1965-69; by the numbers book" shows the DH code translates as a 3.31:1 gear with a Dana (as opposed to the Eaton) Posi-traction rear end designed for Heavy Duty use which was defined as the high horse power 327, or a 409/396 engine combination, or for use under a station wagon.
Your rear end was made at the Buffalo, N.Y. plant on June 21, 1965, and the E under the number means the Eaton Posi-traction mechanism was substituted for the stock Dana posi.
The 3852980N should match the VIN of the car, if it is a numbers matching car.