The rear of my 85 300sd sags, and when lightly weighted the car has significant toe in of the rear wheels.

I have read that replacing my subframe bushings, spring bushings, and diff mount will correct this?

Shocks seem good (bounce test).

I would guess the springs are fairly tired at this point and warrant being replaced.

I would like to see opinions on the HD springs?

Thanks for the help

Welcome to the forum.

We've had plenty of discussions on the W126 sagging in the rear. Search the archives for hours of reading material.

In a nutshell, the following components should be replaced before going after the springs:

Differential mount
Shocks
Subframe bushings.

If the vehicle is still unsatisfactory to you, consider a thicker spring pad above the rear spring. There are three different thickness to select from. There will be a set of nubs on the edge of the pad to determine which one is currently on the vehicle. Report back with what you find.