Perhaps this is your reverse gearbox. It allows you hook up a Chevy drive shaft yoke.
https://www.youtube.com/watch?v=mBkN3CdBtOU
http://roadstercycle.com/Reverse%20gearbox%20for%20motorcycles.htm
https://www.youtube.com/watch?v=mBkN3CdBtOU
http://roadstercycle.com/Reverse%20gearbox%20for%20motorcycles.htm