There are two 3 speed overdrive transmissions that fit Chevy cars and trucks. Both of them use a Borg-Warner OD unit. The Muncie SM-319 has 4 bolts holding the side cover to the transmission case, and the Saginaw transmission has a 7 bolt cover. The Saginaw is an extremely scarce item. Both traqnsmissions require a push/pull cable to lock out the OD when trailer towing or descending long hills. There is also a control relay and a kickdown switch in the throttle linkage that should be scavenged from the donor vehicle.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!