Without spending mega dollars on strengthening the internals so you can run a really high compression and high revs your only route is forced induction. You could keep your block and strengthen the internals to make way for a Skyline head - RB25DET or RB26DETT and use the low down torque of the 3.0ltr with the power of the turbos and greater flowing head.
If you do stay N/A, then fit a Skyline head anyway (RB25DE) for the better flow characteristics and the ability to change the specs of the cams then tune them using timing gears.
Have a look over at skylines aus for inspirating for DET head conversion. Some of the NA boys with RB30DE's are making 130-150rwkw with high comp engines. All depends how much money you want to spend, and how much power you want to make.