It's true. There is no magical hp number that the stock block is incapable of exceeding.
That being said, not all blocks are of equal strength, and unless the entire setup is balanced, blueprinted, modded to make sure there is serious vibration reduction, I wouldn't want to use the stock block in applications of over 450hp.
Too many people put their parts together in a stock block and don't do the additional work to make sure the main webs won't crack from balancing issues. The 5.0HO block is not beefy by any means, and I don't think pushing the limits of what it can take is a good idea with a daily driver.
I would expect that a N/A 306 would probably be capped in the 550-600RWHP neighborhood. Of course, it would be an engine that would cost a fortune, have serious streetability issues, and longevity issues. I like 9,000-10,000rpm redlines too... but I'll keep them on my bike for now.
