If you want to eventually have FI and H/C/I 306, save up your money for a dart block or a ford boss block because that equals block splitting power. Also, your going to need a very stout tranny, manual or automatic. Don't forget that more HP means upgraded trannies, rear ends and suspension.
The big hype of 5.0's compared to 4.6's is that its WAY cheaper to mod a 5.0. The 5.0 has been around for what, 30+ years? The aftermarket for the 5.0 is even bigger than the chevy 350, i believe.
In fact there are few cars putting out 350 to the wheels. Heck, the pontiac G8 GXP with 415 hp only makes roughly 332 rwhp with an auto and 20% hp loss thru an auto. A regular c6 corvette puts out around 350 rwhp.
I wanted to build a 350+ rwhp 347 but that would cost around $5-7K. I have settled with building a stock block, stock crank, stock rods, high compression piston 306 with aluminum heads, ported cobra intake, shorties, custom cam and rebuilt T5. I do have the delema of living in CA and having the biggest SMOG Nazis of all. My goal is to have a fun, reliable street car making around 300 rwhp. If I want to go faster, I'll shed weight or give it a little giggle gas.