you'll need to have the entire 400EX rear end... sprocket hub, brake hub, rotor, caliper, hubs, and wheels.
You'll then need to get a 400EX carrier and commission someone like Stellar or Turbo Chassis to build you a custom swingarm with an offset carrier so your chain lines back up.
I'd suggest just getting a Tusk axle from RMATV for $200 and sell your EX axle to recoup some cash