You wont. PC has a std box and that has 600hp through it at the drags, track etc and its still alive. Bit worn now though.
R33GTSt diff will suffice. Though id recommend a KAAZ 1.5 way. GTR Diff is not a bad option (if you can find a good one), just get it shimmed up a bit to tighten it up. You will also need to change the drive shaft as 1 is 5 bolt and 1 is 6 bolt (but the gtr ones are stronger so its only a bonus anyway)
With your RB30, what hp are you hoping to get out of it.