the stock w126 420sel did not have the hydraulic compensated rear suspension.
so, you can replace the 560sel hyraulic stuff with what was stock original equipment for the 420sel.
some people report that they prefer the 560sel without the hydraulic nonsense.
i consider making the change, but i would want to drive a converted 560sel before doing so.
on another benz board, do a search, because it has been written about. or check out my past posts. there you will find the link to the msgs that discussed this issue.
if you make the switch, let us know what you think after some miles of driving it that way.