Whoah, hold on. Who ever said there was a 400 page script? I don't think any movie script is that long! Even the one that Lucas claims was the big one that had the entire trilogy in it (which was just the aforementioned wookiee and asteroid stuff) was shorter than the final draft, if I'm not mistaken.

EDIT: And as for your other post, I believe it's visible on his bookshelf (or at least it's believed to be this) in the first webisode of the making of episode I documentary. If this is the case, it existed by 1995. Therefore, it would most likely contain notes on everything he'd written by that point. Obviously the knowledge of the original trilogy would be completed along with whatever ideas he had figured out for the prequel trilogy by that point.

Once again, I'm sourcing most of this from what I remember from reading zombie's book twice. I hope I'm not totally fudging things.