I've not heard of the springs needing replacement on these. Shocks can definitely be a factor, the shocks were original on mine when changed at 114K and totally dead (We're talking pogo sticks!). The ass end raised a good 1/2" just from replacing the shocks. YMMV as they say though, mine's a 500SL Euro car and has the anti-squat rear end on it so the geometry will be a bit different from your 380SL.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]