The gearbox story is very complex, there are tens of 370z with standard OEM 7AT gearboxes in US that run absolutely fine, as the one in here, in the other topic. I know guys in US who have done over 60.000 miles on 370z + GReddy TT kit with no issues at all, one of them being Charles, the owner of CJM, one of the most knowledgeable guys in the Nissan world.
I've just finished my first stage, haven't done a great amount of miles since then, but the car performs flawless and has crazy amount of torque and power. 500bhp flywheel and 400lb/ft torque if the Stillen kit can get you that far shouldn't pose any problem for the HR engine.
What are the supporting mods you're planning? FRS, injectors, fuel pump?