I would think it would be as a part of the Gutenberg site, not on a seperate site. That way, all links would be relative. Any broken links would be because of a change in the master directory structure, or an update to one of the books, which would need to be handled anyway (I'm assuming you still want the latest versions of the books).
That doesn't help the problem at all, because what do you do with any images that may be used in the work, such as a DocBook copy of a book or an HTML version of a book? Do you symlink those across the tree also? This is a management nightmare, especially if things move around in the tree (as they are now). It also doesn't remove the space constraints of having the full copy of the work in multiple formats. With the sheer size of the Gutenberg tree, this will rapidly become a full-time job to make sure everything is working right without breakage with thousands of symlinks all over the tree.
If you want it as a seperate site, write the links to point at whichever mirror you want.
Links don't point to remote servers, they point to local resources. Unless of course, these are replicated across some local filesystem and rsync'd from there.
If, however, you want a static copy of the site up to when it switched to the new format, ignoring all new and updated books; then a seperate site would probably be preferable. Or just grab a copy of the 10K special DVD, as it has the original directory structure, and mount it in a web-accessable way.
That DVD is pretty old at this point, and doesn't include the new directory structure, if I remember correctly. David A. Desrosiers desrod@gnu-designs.com http://gnu-designs.com