Locky - I just put a Fox shock on my ZL. I just did the rear skid shock, and it makes a world of difference over the stock Ryde FX unit. The Fox kit runs about $189, with the correct bushings.
Springs will make it stiffer, but not a smoother ride. It's the shock that does the damping. If you want smooth, a weaker spring setting with a better shock will work wonders.
That is - unless you're out riding with that babe my ugly twin brother just married. In that case, you'd want heavy-duty springs!!!

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2000 ZL 550 (Solar Flare, #045/1000)
2000 440 Panther
1980 Jag 3000 F/C
"If it ain't broke, take it apart and find out why!"
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Canaltown Snowmobilers Inc.