Have a look at this thread off one of the US forums, you'll need to sign up to view it though
http://www.300zxclub.com/showthread.php?t=218616
He's aiming for 800hp with an internally stock vg & a pair of GTX3071's, granted US power figures could be considered 'optimistic' compared to aussie ones but worth a read. & have a look through the build section on aus300zx.com, plenty of members doing builds aiming for 350kw so easy to get an idea of what's involved.