There needs to be a little clarification on the GM "Muncie" and "Saginaw" trannies. The SM318 is considered a saginaw trans, regardless of plant. The only true Muncies are the M20,21, and 22 4 speeds. Also, the 318 is not the same as the saginaw 4 speed, you're probably thinking of the 326 or 330 3 speeds, though all were made with the od option. The BW T85 was used in Jeeps as I recall, not GM. Possibly some Ford applications too.
As far as the od itself, BW manufactured the overdrive units for the different makes. Most of the internals are essentially the same. The mounting adapter is the primary difference.
