The hole placement is fine. Not what I would do for an overflow design but it will do the job just fine.
I would consider making the box on the outside of the tank taller. Remember, you need space for a vertical pipe before the elbows/tees that make up the standpipes themselves. Mine is 8" starting directly under the trim and the siphon pipe could probably be a couple inches shorter so a 6" box would be the minimum I would go with. 8" would ensure you can tweek the standpipes to get a fast start without having water go down the emergency before the siphon line purges.
BTW, why the three instead of one long one?
Thank you Rocket. That's exactly the kind of oversight I was worried about. I'll make the external box go up to the rim with a total height of 8".
There's a couple reasons for this design. I'll have 4x 3/4" Sea Swirls mounted to the back of the tank in between each box and on the sides. I didn't want to have to make special brackets, nor did I want them basically hanging over the tank.
The other and most important reason is I have decided to use CustomAquariums.com to build the tank. There is not much info on them here on RC; however, I saw a member received one of their tanks today and seems very satisfied. And they are very popular in the freshwater world. Unfortunately they only sell their H2Overflow which in my opinion is not suitable for a marine tank. Of course that doesn't mean they don't build great tanks, but that overflow for many reasons requires that I build the overflow after market. I would love a C2C, but with the 2 center braces on the top rim it would be impossible. They could probably do a full internal C2C but I intend on keeping the back of this tank free from any and all algae, including the overflow box. With a C2C glass overflow you would see the drains, plus I want a very slim internal box. Nevertheless, I will still end up with approximately 84" of linear overflow if you include the sides of the internal box.
Finally, I'll admit that I chose this builder because of price. They are half of what all the other quotes I got from AGE, C2C, Reef Savvy, and Miracles. Literally half the price and any order over $1,000 ships free. Which then equalled basically a 1/3 of the popular names. I simply couldn't justify spending 2/3's more for a tank.
This company has actually been around a long time. I believe the parent company is Serinity Aquariums. They've cornered a nationwide aquarium leasing market for hospitals, nursing homes, offices, etc. and just recently started selling their tanks to the public. They've also done aviaries for zoos.
I will be sure to post a build thread and report on the quality of the tank when I receive it. I also decided to go with clear silicone rather than their standard black. Any thoughts on this? I just don't like the framed look and stark black lines on the corners of tanks with black silicone.