It shouldn't be that hard to get 600hp out of a naturally aspirated 428FE.
Here's one that I built that made almost 540hp after tuning. This was with 428ci, out of the box Edelbrock heads, a roller camshaft, and a Tunnel Wedge intake. It was also using some Superformance Cobra headers, which were far from a good set of dyno headers. As Barry R stated, when you approach 600hp, block strength is a concern. This one had cross-bolted billet main caps installed.
I'm more of a SBF man myself though. This little Windsor made 586hp at only 5900 rpm.
http://www.youtube.com/watch?v=xFEE-FBsbWU