The O/D unit could be added to your existing transmission, but you need a different main shaft for the O/D extension and your transmission would have to be completely stripped down and rebuilt to add the shaft. It's usually easier to just buy a complete O/D transmission and install that once it's checked out and rebuilt as needed. If you can't find a good rebuilt O/D transmission then I'd suggest contacting Quantum Mechanics http://www.quantumechanics.com/categories.php?op=newindex&catid=11 . The other option is to make a conversion to a 5 speed manual transmission from a Nissan Z car using the kit from Rivergate http://www.rivergate5speed.com/mgb_kit_description.html
Moss also sells complete O/D transmissions and a 5 speed conversion. http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=52461 http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=34771
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