the walbro will have no chance supplying 350kw, its like a joke you even suggest to use one. get a surge tank and 2 x bosch 044's and forget you ever had fuel supply issues
my suggestion for 350kw
350hp low mounted .86 ex housing turbos, what ever brand you like
PWR intercooler
2x bosch 044 fuel pumps with custom surge tank
sard fuel reg
exhast similar to 3" off turbos to 4" system.
dude if it flatens out at 5k check your running 20deg of timing, some people 'tune' them to run on 'aussie fuel' aka 91oct and run them at like 10deg of timing to stop any chance of pinging. that would make it feel like it goes ok when the turbos come on boost then die in the ass just after.
dude they look exactly the same as the pair of N1 turbos i fitted to my gtr,
check the spacing of the oil drain flange bolts,
the genuine n1's are the same as stock, R34 n1s are slightly wider bolt spacing, only 5mm but different.