I don't do single turbo engines in that range, and don't know if this is a power engine, or all out kinda deal. Also, is it in the drag race world or endurance application. And the pump gas thing makes me think more endurance, and not all out. (So take that for what it is worth.) but 400 seems way too big for your application. The thing is gonna be a friggin' pig at lower rpm's and before the thing starts to spool. For a 600 inch drag race engine that wants to run 6000-8000 I would be looking for a good designed head in the 360 + range. Something that wants to run in the 4000-6000 I would want something much smaller. Now as for the turbo side of things, I usually run two smaller turbos on sizes that big, and keep the boost at 10-15 for endurance applications, but even then, I look for a much smaller runner.
Now, here I am making assumptions that this is a street/strip deal or something that needs part throttle and great power, but not an all out drag race kinda thing. So again, take this for what it is worth. I would be looking for something as cast in the 350-360 range, or something that flows big numbers/ported/cnc'd or whatever in the 340 range.