The RB30 will bolt into place of any other RB engine. AFAIK the only difference externally between the RB25 and RB30 blocks is the extra 20mm or so of extra deck height on the RB30. I had seriously considered doing it to my 'line but being too pov to afford it at the moment means pretty much any form of serious modification is on the backburner for a while. From what I've read/heard, the RB30DET gives far more low end and midrange than you'd expect, and helps the turbo to spool up much more quickly than with the RB25 bottom end. The RB25 is undersquare, I think the RB30 bottom end makes it pretty much square, which does go some way to explaining the huge increases in torque that people are reporting.
There are a few guys on SAU (
www.skylinesaustralia.com) that have done the engine work, and meggala's site (in his sig) has some useful information as well. There are a few people who have put an RB25 into a Silvia, but it might be worth getting them to give an idea of bonnet clearance, as the extra 20mm or whatever may mean you'll need to put a bulge or scoop or something in the bonne