The original post identifies the donor car as a "1973 280sl". The R107 280 used a M110 engine. The OP later identifies the engine as a V8. If that is correct, then the donor car is a 350 or 450SL. The difference will be in the clutch housings (bell housings) rather than in the gearbox. The five speed versions of the gearbox in question had a longer tail housing to accomodate the fifth-speed gear pair. At the front of the gearbox there exists the possibility of a longer input shaft for the V8 application. A spacer plate could be used to make up the difference for the six cyl. application.

(The R129 did not arrive until the late '80s/1990)