IIS Redirect of many pages

[SEMMatt]

We have a situation where we want to rename about 550 HTML pages. Pages names will be completely changed. Also the old pages are .html and the new ones .asp. In doing so, we want to 301 redirect the old pages to the new ones - best choice I believe?

All:
What is the easiest way to do this? Does anyone have experience making a 404 Custom Page to handle the redirect? Can you offer any tips.

Thanks!
Matt