I am currently looking for a lighter duty rear spring to install on the back of my Magnum 500.

Does anybody know about an optional rear spring
(in the shop service manual) or if 425 or 325 springs are lighter duty and interchangeable?

I am looking for a spring with less rebound for spirited riding on top of snowmobile/dirt bike trail moguls. Is the shaft ride system tuneable? I am already at the lightest preload setting available and never bottom out the rear suspension.