The piece the mech. couldn't find doesn't exist. He put the smaller 75 shaft in the 78 case having the larger shaft seal so he thought a sleave was missing to fill the void.
I'm going to split the case and use it for a learning tool first but would like to make sure I have the right parts so if at a later date we want to put in a bike and run it it will work.

I think it was a gear meshing problem.

DENVER'S CHOPPERS Preserving a Piece of History