Can the 5-speed “Big Iron” transmissions be adapted to the W123? There seem to be more of them available, plus they are more robust than the 5-speed Getrags.

http://www.ebay.de/itm/Mercedes-Benz-5-Gang-Schaltgetriebe-1172611001-8-W114-W115-Strich-Achter-/202112534291