From,
http://www.jbshocks.com/edgeswap/swap.html
Pm jbshocks on here.
You want to make sure that the balance of the machine is still good. In my opinion this means not jumping more than 1 generation backward. So if a sled has a wide stance/long travel front end such as most 97 – 2001 Polaris sleds (older extra 12 equipped sleds as well) an Edge, ProX or M-10 could make a good choice. If you try to install an edge in an older sled it can fit but the sled will be very tippy and possibly difficult to control. If you must improve the suspension on a 91 then you need to think about a front end as well and that is a subject of its own.
All is not lost for the earlier sleds. If you have one of the older indy sleds an extra 10 can almost always be made to work. The wider the front end the better. Years ago when I installed an extra 10 in a 91 indy I ultimately installed longer radius rods and longer shocks to balance the sled better.
Many people have done Polaris suspension swaps into other brands of sleds as well. But there are other things to consider so be careful. Personally , I have installed an edge suspension in a 97 SkiDoo 500 Deluxe and it was by far the easiest swap I have ever done because no support plates needed moving. I simply drilled 4 new holes. Many others have done Edge into Yamaha.
Forget the extra 12 that you bought, got free, found on the side of the road etc. It is the most difficult to swap and probably the least desirable. People have swapped them in but I wouldn't and don't have the specs. If the sled you are planning to put an X-12 has the right balance with the front which it probably doesn't, you would be better putting an edge in it.