Stock internals, gearbox and diff are good for 500 rear-wheel horsepower. My brother's R33 GTS-t was making a smidge under 500rwhp.
All it had was GT35R, 800cc injectors, coilpacks, HKS cams & exhaust manifold, ext gate, custom exh system, custom intake manifold, XF falcon throttle body, custom FMIC, Power FC and maybe some other things, i forget, was a few years ago.
That was running 22psi and in 12months of daily thrashing it never ever broke
RB25's, RB25 boxes and diff's are bulletproof unlees you want to go mega HP.