Hi all. That fitting does come off. It is the speedometer drive that has gears that mesh with the rear transmission yoke. But I wouldn’t bother with it for a leak like yours. Mine is stripped and so are the teeth on the rear yoke so at present I don’t have a speedometer working.

Doug

1954 Chevy 3100
One problem solved doesn’t seem to shorten the list
Montgomery, AL
In Project Journals