That question couldn't be more vague...but ls's are definetely not made for the top end. If you want power, turbo it. It will cost a lot to get a b18b good top end power if you are going the NA route. You'll need a new intake manifold, cams, valvewprings/retainers, and a ported and polished head. Nothing in the ls head is made to flow well.

Most people choose to boost b18b's.