I had a buddy with an 06 900, he replaced the Sidehiller II's with the Polaris Grippers. If I remember right, they bolted right on. I just went out and measured my 06 with the Sidehiller II's and my 07 with the Grippers and both skis are 3 1/4" at the spindles.
I really like the Grippers, they do a great job of flotation. I have a couple of friends that had 'em, went to after-market, then right back to the Grippers. The nice thing, even new from Polaris, The Grippers are less spendy than some of the after-market, and you should be able to find the Grippers for even less on Ebay or Craigslist.
My wife likes the Sidehiller II's, the dual runners decrease darting over the single runner Grippers. She doesn't ride off trail, so they work fine for her. If you ride lots of trails, you may want to stick with the Sidehiller II's.