My '68's factory original springs only lasted until 1976. At that time it was bottoming out often. That GM replacement set lasted until 1987, when I did the second replacement. A peculiarity is that the factory original springs for a 400 standard suspension was a five-leaf spring and a heavy duty option was a four-leaf. Note that the car never rode great but it isn't a Bonneville. As mentioned, shocks can make a big difference, I did three or four rear shock replacements over the years. Currently I have a set of those "correct look" spirals repros on mine now for an originality look. As mentioned, a more modern robust shock may offer a better ride.
Be sure you have the two axle bumpers (within the wheel wells) and pinion bumper (above the pumpkin). They won't affect the ride but limit harshness and perhaps damage when bottoming out.
I don't see the exhaust system in that photo. Edit: never mind, it looks like a loose tailpipe on the left side. Maybe it's being installed while on the lift.
