Just be happy making 400rwhp with ported 243s, a healthy cam, and bolt-ons, then you can spray 100-150 on top of that. You'll have enough fun breaking shit and trying to get that to hook.
Now if you've got your heart set on 500rwhp and doing it N/A you might want to look into a stroker of some sort. Your stock bottom end is capable of getting there its just next to impossible to do it N/A, you are limited by a few too many things. Dart heads are nice, but are designed to be further ported for maximum performance. AFRs are good too, but for the money TrickFlows perform better (see Gearhead's post). Not to mention that there are a lot of heads that will not work on an LS1's smaller bore (3.898) as compared to that of LS2s and LQ9s (4.0) or bigger LS3s (4.065) and bigger. You want to make big power N/A and there are other limiting factors, so if you're going to build a stroker its much more worth it to start with a bigger block. Just my 2 cents. That would be most people's reasoning and why these options are so popular.