hmm... hopefully you know the science end of this better than your post lets on, otherwise there's is no way you're getting to 400whp, short of throwing a lot of cash and your ride to a shop and just paying them to do it
If 400 is your goal, then the 255 HP pump is the one u want, and you'll most DEFINITELY need an adjustable FPR. Don't forget a fuel pressure gauge, or you won't know what your A-FPR adjustments are doing.
for 400 you'll also need bigger injectors, AEM EMS or GReddy eManage or PowerFC, larger turbo, FMIC, better pulleys, good plugs to match your new output, intake, lots more gauges, intake plenum, various piping depending on the turbo u pick, your choice of electronic boost controller, maybe 5lug conversion if u wanna be anal, turbo timer, 3" straight thru exhaust system, header of your choice, 3" hi-flow cat, upgraded radiatory (koyo 3core alum. etc), clutch that can hold the power... might as well get a lightened flywheel while it's apart, LSD of some sort to actually use the power u put out, wider rims while you're at it, internals would be a good idea, but not necessarily a req...
You can splurge in some areas to forego others, and vice versa, but you get the idea.
i'm sure I've left out a lot, and there are definitely build choices to go in various directions, but others will fill in the blanks.
It takes a lot of $ to get down to 10mpg, and u gotta pay to play.... and we haven't even gotten to the footwork yet :P