I think just a weight change and possibly a primary spring change will be all you need. I would see what cat recommends in your manual, and is should be listed near the belt. With a diamond drive a gear change is going to be a pain in the A$$. The F-6 may be geared too tall but it will be not affect the running of the of the sled too much. Even with out gearing down it should hit peak rpm by just clutching the Primary and sec for 5 to 9000 ft. The f-6 will give a Errrrrrr sound when you grab a handful from a dead stop. The key is to keep your R's up in deep snow with tall gearing.
The Cross fire should just need a weight change, 700 has plenty of torque once on the pipe.