Originally Posted by
Ian S
You could probably use 350lb springs with that weight. But you'd need custom valves shocks. You need the rebound setting more resistant, or it'll bounce due to the extra spring force which needs to be a controlled release of the spring compression. And the bump setting needs to be softer as you have harder springs and you'd don't need the shocks also preventing the wheels from rising over bumps.
Not the cheapest but probably the easiest to get right would be for you to get decent shocks with separate adjustment for bump and rebound. 'Coilover' springs are (were?) relatively cheap and easy to swap until you find the optimum spring rating and height for your vehicles mass and configuration.
To get a softer ride, maybe use tyres with taller side walls?