Well since you have an Impreza and not a WRX to start you have to swap the dash harness and ecu/cluster. You might as well swap the entire interior while your at it.
If you had a wrx it should be a direct swap. (motor wise)
If you are planning on doing the trans & rear diff best bet is to swap hubs/knuckles/brakes as well. otherwise you will have to get some custom axles. Literally have to swap EVERYTHING from one car to the other.