Redirection

Our company's website was recently rebuilt from a collection of many HTML pages assembled over several years to a new mysql-based site. We used to have a large collection of numbered files in a directory called specs, so the url was site.com/specs/6413.htm, for instance. Now it needs to be site.com/display_page.php?p=200&s=6413.

Is there a way I'm unaware of to have it direct misdirected viewers to the correct page? I have access to the 404 page, but I'm unable to rename it from it's default .html and create a dynamic page. Any ideas?

Thanks.