www.nengun.com
www.greenline.jp
not sure if theys hip to the states... but alot of the guys here in Aus use them for skyline parts...
for big power i'd probably go for a single, you'll need someone to custom make you a manifold, surely someone can do it locally to you?
Don't forget with that kind of power you're going to need lots of other bits, ecu, bigger fuel pump, bigger injectors, forged enginee internals etc....