The xtra-12 is not what you want. It's a falling rate suspension, designed to give a very plush ride on the trails. You may be able to get an Edge suspension from a later RMK and fit it in... but raising the back without doing the front can really mess up the ride quality.
I had a 97 RMK 700 and went to an Edge 800, the Edge was a lot more capable in the deeper snow. An Edge 700 is pretty much the same engine you have, only with power valves, so it should be as easy to fix and as durable. It's something to consider...