No, see I was going to say that you should look into getting a GTR rear cradle (coz the driveshafts need the centre to use) but its not really the axle you need to strengthen, its the CV joint. It may still be worth looking into.
The problem you have, is the running gear in the S1 is the same as the R33 GT-4 and they don't have the same "meatiness" about them as the gtr, because the R33 GT-4 was only ever released with the NA RB25. To prove this, just compare a front driveshaft from a GTR to a stagea, and you will see that the stag ones step down in diameter before the diff, whereas the GTR ones don't.