lol terrible... i have about the same power as you and last tank got 420km.
Before the tune it was doing about 500km per tank, so extra 100kw in the tune and only lost 80km per tank, pretty good.
The engine will need to be from a stagea or a gtr or it will not fit. Well the sump wont anyways... If you think the engine and labour is going to cost you 5 grand your underbudgetting pretty badly.
I had the same problem when changing plugs. Didnt take the crossover pipe off just shoved it aside and it made a hose pop off. Look near your bov and you should see 2 hoses going underneath, one of them on mine came off and it idled like shit.