Racecarl pointed the way to the New Process A833/89mm/MY-6 GM 4 spd overdrive. Used only on 81-86 GM trucks, it is a little hard to find. I just installed mine behind a 66 250 in a 52 Chev 1/2 ton with no problem. Not on the road yet, but I don't anticipate any trouble.

See it at
http://www.homestead.com/chuckthetruck/page12.html