For starters it is risky and dangerous to machine the diameter of a disc down just to save a couple of dollars not to mention no warranty if they explode which i have seen happen.Has the rotor been cast for this to happen and to take the risk.
R34 GT rotors are 296mm front
R34GTT rotors are 310mm front
R34 GT-R rotors are 324mm front
DBA rotors are available for the Skyline range including the R34 GTT 310mm where as RDA do not do this particular disc.You can buy std 4000's rotors instead of there slotted if you want.