Here's how to duplicate one of your blogs, I mean sites:
- Download your post template and export your blog.
- Make a new blog - that'll be the copy.
- Upload the post template and blog that you just downloaded.
- Manually set up the "layout" and pages... this is the lame copy-and-paste part that escapes the smoothness of the rest of the process. In the "pages" part at least, your pages will probably all be there as drafts, and you just need to publish them.
Then you're good to go crazy on that template. I'm going to use this opportunity to take all the useless Blogger code out of this site, so it doesn't have to be so bloated with unused HTML and CSS to make it disappear or overwrite their lame CSS.