Bolt-ons
edit: my two cents BTW, build your car up to support a S/C dont just slap one on, and if your gonna get heads get som patriots or something a little more higher performing than stock ls6 heads.
Just bolting on stock ls6 heads isnt going to yield much.
I think the first thing you should do is learn about camshafts and think about replacing the stocker. The stock heads flow pretty good, so even with a small or mild cam, you can grab another 20whp.
The first boltons I would advise are intake and exhaust (Lid, catback) Easiest, cheapest power you will find. From there, If you were to add a cam and a set of Long tube headers, you could see another 50+whp from where you are now. Then adding ported/polished LS6 heads would prob give you another 30+.
If you have the money for a supercharger setup, go for it, especially if smog is a concern. 450+whp is very common on stock internal engines. Most people go the route I described because it is way cheaper than forced induction.
hope that helps
btw, do full exhuast ( catback, LT headers, ory ) and intake ( lid, pre01 cars ls6 intake ) and UD pully before you think about doing a cam. stock heads will be good.......for now.

