The 8 3/4 in my sons 68 Dart came out of a 1965 Dart wagon 4 speed with the towing package. It had 3.54's Sure Grip.
Was it Factory or Dealer installed, I do not know. In about 1976 I bought it from a man in his 60's that bought the wagon new to tow a small Airstream for him and his wife. He had sold the engine and trans otherwise I'd had bought the whole car but it was a Flintstone car. Frame and body was 80% rust.