You might want to google a few images of the MK3 diff as I think it can be mounted from the top, if you remove the stubby cast ali arms it is effectively hung by 4 bolts. Although you will still need to make a SOLID mount for the nose of the diff that bolts to SB4. If you still have your chassis set up for the Sierra diff a section of 3-5mm plate welded to the under side of RS11+12 would be the way I would go with it.