[Tigga]

Do you have any need for a seperate header template for your index? If not, you can just edit your index template and replace $index_header with $header. Then you would need to edit your header template and change each link there to point to the full url to your forums.
If you do want a seperate header for your index I would recommend copying the content of your header template to your index_header template, and then changing the links to point to the full url to your fourms. Oh, and a little tip... You can just put $bburl in front of each link instead of the actual url. That way if you ever change the location of your forums you won't have to update those links again.