here's a little FYI for you before you attempt this swap.
you'll be MUCH better off keeping the VG30ET (if you have one) or getting a VG30ET swapped into your N/A Z31 than going with either the RB20 or RB25 swap. why?
for one of the more obvious reasons, the displacement. it's not that much of a difference over the RB25, but it's a definite advantage over the RB20.
second, the RB20 only makes 215hp, i believe. i don't remember, but it's in that vicinty, maybe a little less. and the RB25 makes 250hp. now, the VG30ET makes about 240hp, that's only 10 less hp than the RB25, but the displacement makes up for some of that.
also, remember that the VG30ET is a SOHC, single turbo, SOHC engine compared to the RB20DET or RB25DET, which are DOHC, single turbo engines. the VG engines are very tough and rigid engines to work on, and with the Z31, you have a bit more room in the engine bay, compared to the Z32s.
also, the RB engines are pretty well-known for the #6 cylinder going out. it's pretty weak from what i understand, and usually needs to be replaced or the internals are recommended to be built up. that's what i've heard.