I want to put a turbo on stock engine I want to make 270 hp but I don't know how much boost do I need?

270 Hp to the wheels isn't possible on a stock internal KA, I don't think. From what I remember, 10PSI is the max safe boost you can run on a stock KA. That should make 230hp to the flywheel. So it should be 200ish to the wheels (15% drive-train loss).

huh? not quite.

replace the headgasket and a twin cam KA can run as much power as you want on stock internals.
cory (nissanfanatic here on AF) has already hit 450+ whp on his internally stock daily driven KA, with almost 500 on race gas. but hes smart and knows how to tune correctly, so results may differ.

270whp = fuel pump, injectors, big t28 setup, intake, exhaust, clutch, and tuning. you may not even need a HG for that power level but just to be sure i would replace it with a multi layer steel gasket.

oh and dont skimp on the tuning. its what makes the difference between a strong motor and a paper weight.