You might as well use 404, as redirecting many pages back to the home page with 301 will give you no SEO benefit. The idea is to redirect individual old pages to individual new pages, unless you do that the SEO benefit is null.
If you really want to throw a 301 for every page you don't have on your site then do it in Perl or PHP using the guide here:
http://www.seoworkers.com/seo-articl...tml#modrewrite