isnt the 123 front end basically the same as the 114 / 115 / 116 / R107? i.e. a massive subframe with everything bolted to it? (i dont have my epc here)
Im doing somthing similar - and all my measurements show that the 6.9 litre M100 engine (from a W116) will fit straight into a W114 (max engine size 2.8).... Im not saying the exhast will fit though
hmm - did a quick google - http://www.diymbrepair.com/ suggests that the 123 is a mini 126 - so in theory a 560 lump should just bolt in...