Look at the Polaris spec for the RMK in that year and that would be a good place to start for clutching and sprockets in the chaincase. Track rail extensions are not as robust as a true 144" suspension, so if you are hard ditch banging, the extensions may not hold up well over time. You will also need to think about a tunnel extension, as the snow flap will need to be placed at the rear to help keep the cooling system cool (as well as not hitting anyone riding behind you with chunks!)
Doing a whole 144" suspension will require re-drilling the mounting holes for the suspension, so if you go that route, make sure to get exact measurements from the donor sled. Even a little off may compromise ride quality.
Welcome to the forum!