It strikes me that there cant be a huge amount of difference between a 280 block and a 230 block. Mainly the weight difference is due to the extra equipment that the majority of 280SL's would have over the majority of 230SL's. Obviously a 230SL manual, with no PAS would be lighter than a 280SL auto, PAS, A/C and whatever else might be there.
Basically the 4 bearing 230SL isn't quite a smooth or silent but feels a little racier when it comes "on cam". I think that a 280 engine, providing you have the "hotter" cam, will give your manual equipped car the best performance combination !
Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor