As long as it has the z-axle.. it should be pretty easy (please correct if i'm wrong).
The major aspects will involve the hub assemly, as well as the handbrake cables. You may need to replace the cables as the actuator in the hub is very different. Instead of acting on the existing brakes like a drum does, the discs are completely seperate from the handbrake assembly which are small drums inside the disc.
Most likely the whole upright assembly that contains the disc will need swapping over to be able to carry the caliper mounts etc.
If you can get hold of a full assembly from the 320 it shouldn't be too bad.. full days work.
Yes... i'm a total car nut! Why else would I upgrade my mower to rev to twice its standard rpm!