Hm, this was easier in older versions. In the sidebar of the admin in the "write" screen, you could simply choose a template. Maybe in newer versions this option is only there when you have different templates. In any case, try the options from my previous reply:
- make a new template with the code in it. you can just open the index.php, put the code in and save it as "index2.php" or whatever name you have in mind;
- "manage" that page that you want to use the template from and see if you can choose that other template (index2 or whatever you called it).
If that doesn't work, I'm out of suggestions.