I would NOT use an override, but rather a pre or post.
They changed the name of the hook in 2.1.x from what it was in 2.0.x so depending on your version, you may have different filenames.
…hooks/orders/confirmattion.post.tpl for 2.0.x
…hooks/checkout/order_confirmation.post.tpl for 2.1.x
The my_changes addon is enabled by default. So unless you’ve specifically diabled it, it should be active.
In v2.1.2, in Design–>Blocks–>Order landing page, I created an html block and posted all the conversion code there. Not sure if your version has the same option, but seems like this would be a good approach.
I haven’t tested live yet, so…disclaimer.
It’s a great approach if you want to redo that every time you upgrade. Why not use the hooks provided for that purpose?