Are you looking for 340 crank HP or wheel HP?
Crank should be easy to come by, but wheel will be pushing it.
For crank...
I'll quote what I told another Z owner about modding his Z...
"I would get the following...
Tomei 268 Camshafts
Injen Cold-Air Intake
Injen or Borla Cat-Back Exhaust
Nexus or Crawford Headers
Nexus Test Pipes
Altered Atmosphere Intake Plenum Spacer
2005 Nissan OEM Lower Intake Plenum
Unorthodox Racing Crankshaft Pulley
Technosquare Full-Spec ECu Reflash
I'm not sure about what else.
Some custom things you may want to try. A Moreso fuel cooling can/tank. Here's what I would do, lighten the Z, carbon fiber hood, lexan glass in the stock hatch, take the tar out of the back, take the spare and jack out (unless you need it, if you don't want to get AAA), get a Hawk racing battery (maybe it's called Hawker? not sure, and I never found out if the performance of the battery changed or not) and i'd relocate it to behind the seats. And mount the fuel cooling can/tank in the compartment where the battery used to sit and then hook the fuel lines up to it. For every 10 degrees you cool the fuel, you gain 2-4% hp, use the fuel cooling plus 100+ octane (which the OEM Ecu will adjust timing) and you should see some pretty good gains.
I wish though...that someone would step up and make a more simple design carbon fiber complete front bumper replacement, rear bumper, and side skirts. (CWest just ain't cutting it, Wald side skirts replace the stock side skirt) And also CF fenders...every little bit counts."
Some of that stuff may not pertain to you. :rolf2: