According to my dictionary of American Cars, the only rear wheel drive engines available were the 4100 and the 350 diesel. The Buick 6 was last used in '82, and Cadillac stopped offering the 368 in 1984, when they discontinued the Fleetwood Limousine.
Of course, if you don't like the engine, you can swap in almost any Cadillac, Oldsmobile, or Chevrolet power-plant (from the 500 to the 305; it all fits).