if youve been to the projects thread, a guy building the most unique engine build about has a Z tune as his daily, 20 pages and well worth the read
http://www.skylinesaustralia.com/forums/topic/355547-r34-gtr-build/
probably to do with the power delivery/whack of torque vs how well its tied down and not so much peak power/quality of build when shredding tyres on dynos
(this is not meant to detract from it being a tough motor, just saying)
just make sure you know whats going in the motor, there are lots of tricks, but it all comes down to what you want out of it before you need to spend X amount of dollars on a build that doesnt really require it
Is a tute on here somewhere, also the manual, because of gravity I think you can just crack the bleed nut and let it bleed down making sure resivor stays full
what they are saying is that the GT-SS twins are tiny and apart from stock, you wont find much else more responsive,
hence why you should look at the whole setup/tune.
Like Zebra said, if specified the manifolds are fabbed to suit, i think all 6boost manis are made to accomodate rb30..
http://www.skylinesaustralia.com/forums/topic/219307-bonnet-wont-close/