Help 66 rear suspension

If the springs are original, its common for them to sag at differing rates. The tab could be a factor.

Air shocks are a 1970s fad whose time has come and gone. I would remove them as soon as practicable...if you look at the upper shock mounting point, you will see that its just the car shell steel, through a small hole(s) with no reinforcing plate whatsoever (~14 gauge IIRC). This was never intended to take the weight of the cars back half, which is whats happening if you air up those shocks past the spring's load (which you ARE if you are compensating for saggy springs). I have seen the mounts ripped out by air shocks before.

Look into new springs and real shocks. They are quite reasonably priced nowadays.