Earlier w116 300SD uses a rod and not bowden cable to help regulate tranny shifts, hence ye dont want the earlier trans in yer modern w126 300SD.

For my money I'd toss yer existing engine into the w116 car (all other factors being equal) knowing the earlier Benz outhandles the later version, plus the w116 being most classic of the 300SD series. Extremely controversial too, USA *only* got em - makin these the holy grail of European desirable vehicles. Am kinda surprised more Europeans dont snatch em and ship em overseas.