This is going WAY back in my memory (pre 5.25" floppy!) but I seem to remember that "in the olden days" 510s had to use really stiff rear springs to control the wacky motions induced by that semi-trailing arm arrangement that they have.

The thinking was that any potential benefit of a more pliable rear end were offset by the toe/camber changes that resulted over anything like reasonable travel.

It is never going to be as simple as looking to other makes/models of car - sorry!

(Big FWIW.)

Kirk