This is probably way off, but shortly after I bought my 69 I noticed it too was sagging on the driver's side. I replaced all suspension with the hotchkis TVS system and it was still sagging. It was not until I cut out the rear panel and replaced it that I noticed the sag had gone away. I hope your unibody is not slighly swisted like mine was.
If it makes you feel any better, I found my axle tubes on the original rear end were dented on the top where they had hit the rear frame rails cause someone had probably done their best dukes of hazzard jump with it...I'm pretty sure this is what caused the twist in my unibody.
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1962 Nova, LS1/ T56, DSE Mini tubs and subframe connectors, Church Boys 4 link, Viking coil overs, C6 brakes
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