That is a Borg Warner overdrive. All you need is a cable to operate the lever. When you pull the cable, it will lock the overdrive into direct and it will perform like a normal 3 speed. When the cable is pushed in, it will allow it to shift into overdrive. The part in front of the overdrive should be the same as what you have now except it has a mainstaft to mate it with the overdrive. If you take the shifter cover off the transmission, you can see the general condition of the gears. However, the wear parts will interchange with what you have now. As for the overdrive unit itself, the main thing is that it didn't run out of oil. Parts will interchange with any vehicle of the period that used overdrive. The only thing you would probably need for sure is the relay. They are on eBay for about $100 all the time. They don't have to be from a Chev. The kickdown switch can easily be substituted with a toggle switch.
