Here is what Hollander cites. 1955 through 1968 early can be interchanged as a unit, but you will have to have an open drive shaft. It cites the GM as well as Borg Warner part numbers for the main drive gear as well as the cluster gear. None of the internal parts will work in your closed drive shaft transmission.
https://photos.app.goo.gl/iQsnfNF4ZnpeKfyDAYou are probably correct that it was produced in 1964 which means that you have the later main drive gear which is irrelevant because none of this will fit into your case.
