If you carry heavy loads, I'd avoid sportline springs and sport shocks. Starting off with a lowered stance probably won't help.

Urethane bushings are available but I hear they're ridiculously stiff for everyday driving. I don't know if a urethane strut mount is available.

There are a couple of bushings on the aft end of the diff that are probably shot if the car was driven with harsh shifts. Look under the car and see if the bolts going into the diff from behind are centered in their bushings. Most likely the left bolt has bottomed in the bushing cavity.

There is a carrier bearing between the rear knuckle and the lower control arm. It's not obvious from a casual glance there's a bushing in that joint. That bushing is notorious for causing an embarrassing creak when you put weight in the car. Sounds like the frame bending.

Sixto
87 300D