If you swap out the h/c/i, you should be flirting with 450 easy. I was told that if I swapped out my heads for Edelbrock 5.0's and an F cam I could expect around an 80 rwhp gain. The tuner recommended the Edelbrock 5.0's because they will allow you to keep the stock valve covers and the stock intake.
Methanol injection is a viable and proven way to increase power and acts as a safeguard against detonation. They used it on WWII fighter planes, so the idea has been around for a while. I actually have a meth injection system being delivered tomorrow.
Other options for adding power without the expense of swapping the h/c/i would be to run E85 or racing fuel. E85 is cheap and has an octane rating of 100+ but the downside is that you have to run about 30% more fuel, which could mean you would need larger injectors and fuel pump. It also lowers your mpg's a good amount. Racing fuel will net you the greatest power gains with no additional modifications required, other than needing to have your car tuned for it. The downside is that it can be expensive, around $7.00/gallon, and it may not be available locally. I have been seriously considering running racing fuel because I don't put that many miles on the car every year and there are a lot of places around me that sell it.
I was told not to bother swapping out the stock fuel rails because they will handle around 500 hp without a problem.