How to Add Order Number into Invoice's Header.tpl

[Precision_Sam]

Hello, 

I am trying to customize the invoice template (PS 17). I'd like to have righ up corner of the invoice showing:

Order #:

Order Date:

Invoice #

Where should I modify if I want to add {$order->getUniqReference()} into header.tpl? What is the better to do that?  Any suggestion is very appreciated!  Thank you!