Charlie has good suggestions. Keep in mind too if you look into something like the LS swap (which you can get as crate engines as well), you've got exhaust, intake, lots of other differences to take into account which will get expensive quick. If that's not a problem, then go with something like the LSX454, which is around 600hp on pump gas.

If you want to get into serious power and/or get the most from what you've got, you're better off staying with fuel injection and getting tuner software. It's not easy, but if you learn it and the tuning process, you'll get the most out of your investment. If you want easy and pretty good on power/efficiency, go with a carb.