That is correct, I am currently trying to install a w126 into my 84 wagon and there are definitely differences. the back arms that bolt up to the ceiling are not in the correct place on the w126 (not sure about the w123 frame) as well the wagon appears to have a brace that the w126 frame does not account for. I am in the process of re-removing it and making the proper adjustments to the w126 one so that it will fit, its two extra things beyond the power to manual conversion.

Did you use a w123 frame??