His "spillway" is on the outside and he has drilled holes to let the water in but, it's on the shorter "side" section from front to back. I was considering, as you are, about doing this across the longer back section of the tank. However, because acrylic will bow, I was affraid that one of the seams might crack when the water was added. Thus, I opted for a coast-to-coast overflow on the inside. It's definitely a compromise in that you are losing some of the volume on the inside but, I think it's a more safe bet. Just some things to think about. Let us know what you decide!
We have never liked the idea of overflow boxes on the outside of the tank because if they leak than well you know where the water goes. Acrylic or glass for that matter will bow when under pressure. Our thinking on that is pressure pushing against your overflow box inside the tank O K pressure pushing against the back of the aquarium may cause an external box to separate from the back. I also think by cutting those long slots in the back of your tank will make that area for the tank more likely to bow as you will be taking away material leaving small strips of acrylic between the slots. I have seen aquariums with external boxes that never leaked, I have also had to repair external boxes that have pulled away. We discourage this design and do not warranty it if we build them.