Dr Diff Master cylinder

Looking to get this for my 67 Valiant 73-up disc front, rear drums. I was going to go with the 15/16 bore. Do I HAVE to get the adapter? Its an additional 25.00, just curious if its something I need to have.
*EDIT. Nevermind, I see its to fit the newer style MC..
Mopar Aluminum Master Cylinder Kit