Adrian,
Not sure that you'll be able to use the GTR size rotors on GTS25t calipers, because the rear caliper for brembo takes a 22mm thick rotor versus a 18mm thick rotor for the Sumitomo calipers...
I don't think the GTSt calipers have the bridge thickness to fit the 22mm rotor...
These guys do a 320mm x 20mm kit for the standard caliper that may work... but means you have to run their particular rotor.
I've sent them an email regarding the brackets for the GTR33 rear rotor -> 350z rear rotor conversion... hopefully their kit doesn't change the caliper offset - otherwise it will mean you have to use their particular rotor as well.
http://www.biot.co.jp/parts_brake/offset_k...etkit_rear.html
will keep you poseted to their reply.
cheers, marcus