Do your search for Chevrolet overdrive. Most sellers don't know that the OD part of the transmission is BW and the main ox is Muncie. A BW search will usually only turn up Ford stuff because Ford's OD transmissions had BW front boxes with the BW OD unit behind them. Saginaw was only used for a year or two in the late 60's and they are very rare. I stumbled across one at a swap meet a few years ago and the guy didn't know what he had.
