It would be simpler to download this tarball directly, mount your new partition, and then untar the downloaded file directly to the new partition. (the thread you quoted has instructions for this, too; that is in fact what BrentDC was doing)

Mounting the image file and using tar to move the files is fairly messy, and you only need to do that if you've been customizing your Debian system and you don't want to lose your changes.