You need custom brackets to adapt the existing rear calipers and the factory bracket must be hogged out to accomodate the wider rotor.
Warmonger had a group buy for this conversion. It was a sellout. You might be able to find someone that bought the kit and has decided not to use it if you check out the thread in the group buy section.
This kit is best used with either his front kit for using the Focus SVT front rotors with the original calipers or adapting the Focus SVT calipers with the Focus Rotors.
There is also a thread about the installation of Warmongers kits here in the brake forum.