There is a thread open on this already.
MB shop current thread
The diesel engine may be a bit heavier than the SBC - to help the weight bias a bit consider installing 240D springs.
BTW, ignore the purists. The W123 chassis is a really good unit, and will appreciate any powerplant you care to install. Just don't tell anyone if you plan to put in an English or Italian engine.
Personally I think the engine/transmission from a Buick Grand National would be a marriage made in heaven for the W123.
