You can use the bigger pulley in the rear if you are using the front one way. I would just use the two 15 tooth pulleys as it will let belt be a little freer and 03 are just not as efficient as the 04's are. You will find that the 03 will not do as well in the GT3 class as it would in GT2 (stock or even GT1 (mod). I run mine in the TCS races in GT2 and was actually leading and could have one if I wasn't taken out but that is another story.
Our track actually runs a GT3 class every week and the 04's and the occasional EVO1 or EVO2 are fast. The 03 just don't have a chance with the silver 540 in their and take it from me I love the 03 cars and wouldn't run a 04 so unless it is stock or mod your poor son might take a few lumps against the 04's. Good luck if you do run GT3 as I would like to know how you fair against the 04's as they are also a ton lighter.