Isn't there a lip that is turned down on one side of the mount and overhangs the frame cross member? I would think that would prevent the reverse mounting. I know my BB mounts have that turned down lip.
My 67 BB fan sits about 1/2 the blade top curve into the with the end of the shroud. I though the SB was similar though the BB and SB used different P/N spacers and there is only a 1/4" difference in depth between the two with the SB having slightly more depth reach.