You don't have a full load on your springs yet. I can see you have not installed rear glass. Are seats in, door panels, engine ... etc?
Leave all bolts loose when attaching rear axle to springs and spring ends to rear shackles etc. Don't tighen up bolts until the car is resting on the rear axle with a full load.
Engine Test Stand Playlist: https://www.youtube.com/playlist?list=PLwoxyUwptUcdqEb-o2ArqyiUaHW0G_C88 restoring my 1968 Firebird 400 HO convertible (Firedawg) 1965 Pontiac Catalina Safari Wagon 389 TriPower (Catwagon) 1999 JD AWS LX Lawn tractor 17hp (my daily driver) 2006 Sequoia 2017 Murano (wife's car) 202? Electric car 203? 68 Firebird /w electric engine 2007 Bayliner 175 runabout /w 3.0L Mercuiser__________________________________________________________