I remember going thru that w/my C3....I think it's a fairly common issue with the transverse leaf spring set up at the rear, both sides not being level.
I know I evened it out on my 72 after a few trial & retrials.....using the nuts on the spring bolts &* going up or down to get the bumper height even L to R. My Z is slightly off, I gotta stare at it for a while, maybe 1/2" or less. Jacking it up , stands under the frame, jack the spring end w/a puck on the jack pad and soft wook block up against the spring to unload it so you can move the nut, let her down, go drive around, re-measure & if not good repeat....just too tedious so I say lower it at the same time w/longer bolts and new bushings so it's worth all the trouble...
I think you're right about the spring bushings being shot, or close to being shot...MHO it's age as well as miles....I'm not a fan of poly, as I think for street use the car is stiff enough although the spring bushings might not make a difference if they were changed to poly with respect to stiffness/harshness. I know when I redo all my bushings I'll use mostly stock stuff as I'm not unhappy with the results that they give in my car. Maybe some Heim-jointed sway bar end links and maybe poly rack & swaybar mounts would be something I'd seriously consider, but the rest would proly be OE type stuff.
Tom