Those turbos are tiny for the power you want, A/R .64? Please.
For 600kw at the wheels you're looking at some serious horsepower, and a built engine, you'll have to fortify the top end with various JUN / Tomei / Whatever parts and maybe even the bottom end depending on what you're going to be using all that power for. Remember you have an RB26 not a 2JZ lol.
Secondly I wouldn't buy my turbos off ebay, I'd buy from a registered Garret dealer where you have things like warranty.
If I wanted 600awkw from an RB26 I'd get 2 GT35R's with .82 rear housings from a registered Garret dealer, like Horsepower In A Box.
Then I'd find a workshop willing to tune such an animal and an ECU that would be reliable at that level of power, you're going to need a custom fuel setup, at least a 2.5L surge using something like a Carter Gold lifter pump and 2 x Bosch 044's with return lines.
I wouldn't bother trying to bore out your engine to a massive 2.8L, is that even possible? Instead you could purchase a stroker kit.
This is all going to cost big dollars, I wouldn't undergo this project unless you had at least $25k and a workshop that you could trust.