You have to shim for endfloat using extra gaskets between the middle and inner castings, soons like this should fix it. Read the how to in the twch section if you haven’t already, it’s very good but you need to read it more than the once.
Good luck!!
You need to shim each shaft separately to give .005"-.010" end float. If you scroll through Neal's page here (in technical) to "gear box tips" he explains it. http://www.wdbsa.nl/BSA%20M%20type%20gearbox.htm