I wonder if that rear wasn't a Chev only thing meant for vocational use such as a mixer. GMC only used the corporate rear up to the 370 model. The 400 and larger used either a Rockwell or Eaton. Eaton had a similar looking contraption, but went to a through shaft like Rockwell. I have GMC books from 1955 through the early 60's from the 100 series up to the 970 series, and I don't see anything like this Chev has.