ok, well i'll point out if you are doing the rb26/30 conversion, you would be using the rb26dett head, which already has the front facing plenum from factory.
gearbox etc, can be spaced down the same amount the engine is dropped, but ive been running my car without doing it with no vibrations, or problems.
Turbo, i suggest whatever power you wish to strive for with the supported mods. Gt2835r or upwards (gt30r minimum), and a longer diff ratio.
25/10 is way too small like my turbo. On boost its useless, just wheelspin.