How to customize 404s in this manner…

I’m in the middle of transferring my website from Nucleus to WP (using RSS). With Nucleus I was using modrewrite to handle files (item-174.html >> index.php?item=174).

One concern I have is anyone who’s bookmarked a page or is coming in from a search engine seeking one of those Nucleus pages, I can’t redirect it to WP because the import process didn’t use the correct post IDs.

So one idea I have is to make it so if a request generates a 404 error, to automatically list a list of possible pages (like, “was this the page you were looking for?”), either using excerpts or just title or whatever.

Any ideas how to do this or can someone direct me to where I can get started?