Unless you want headaches don't put a 400 Carb on! Getting the Jetting right is a big PITA! lol but as billy stated the 400ex a arms but go with the 450R shocks. And I've heard the 660 Raptor rear shock will mount up. You can also put a 400EX swing arm on that will give you about 4" more of wheel base, then you would be able to use a 400 rear shock.
Just because you can squeeze a shock from a different quad in the rear does no make it an upgrade. To use the 660 rear shock you must remove the linkage system on your swingarm. When you do that you are completely changing the geometry at which the shock is designed to work at.
Hey man, my brother had a roost boost on his quad and now it is having all sorts of ignition issues. I can't give you specifics because it is not my quad and therefore not my problem, but they could just be from old age as well. My advice from expirience: sometimes it is better to go with what you have.