At the moment, when a URL isn't found, I'm serving a 404 page which shows a list of partially|fully matched alternative pages that the user is may looking for, without saying that is an error page.
If no any similar page is found, I show instead the Not Found error message.
Basically what I just do there is searching pages that match the "keywords" in the URL and show the list, instead showing the error.
Now, instead serving it as 404 I want to know if I can make improve my SEO returning a 300 Multiple Choices status code instead, which to me seems exactly what I'm doing already.
Will these pages be indexed by Google?
If yes, should I allow that? Many generated URLs may be actually useful to be found in Google SERPs, since that "disambiguation page" will implicitly explain differences between two or more similar resources... but I'm worried of people appending bad words in the URLs and linking to those, just for to manipulate my ranking.