Anyone have any tips or mods to improve ride quality from the suspension of my w126?

I was spoiled with a w140 for awhile, but now I'm back in a w126. I don't expect w140 ride comfort but any improvement would be awesome.

The car is comfortable but you feel every crack in the pavement.

I have a 420sel with a om603 swapped in, but plan to drop a 606 in.

anyone know what model w126 had the softest springs that I can drop in?

also replacing what components in the suspension give the best bang for your buck comfort-wise?

Any input is appreciated.

__________________
1974 240d
1983 240d -sold
1989 420sel with om606
1994 s350 turbodiesel - sold