700 gph overflow kit

If you know someone that can cut glass i have some extra pieces laying around that you could silicon into the tank to make an overflow. I'd build it into the corner of the tank so you only need to worry about 3 sides instead of 4 if you put it in the middle.

 

One of the key factors when building an overflow is the number of linear inches exposed to the water surface so that your tank is getting proper surface skimming. If you put your overflow in the corner, you have one less side to make up the linear inches required for a properly sized overflow. A corner overflow is just as effective as a center overflow, just make sure that it's appropriately sized. Reef Central has an overflow calculator on their front page.

--AJ

 

Well, there's a lot more than just trying to size an overflow box when you're setting up a return pump/overflow system.  You have to consider pump sizing, head pressure, (the amount of pressure that lifting the water to your tank puts on the pump, and how that impacts the pump's ability to move water), amount of flow you want through your display tank, size of plumbing, etc.  I would look at things like this:
1) How much turnover (water exchange between display and sump) do you want?  Personally, I usually go with around 5x the volume of the display.  40g = 200 GPH, 90g = 450, 120g = 600 GPH, etc.  This is just my rule of thumb, someone else may have a better formula, but this has worked for me.
2) Decide where your sump is going to be located and where your display tank is going to be located.  Understanding this, is critical to pump sizing as you need to know how much plumbing you're going to have hooked up to determine the head pressure.
3) Calculate your head pressure.  This is a critical step that many either overlook or don't understand.  Figure out how your sump is going to be plumbed to your display.  How long is the plumbing between your pump and the return?  Are you coming from a basement to a main floor, or just from inside a stand to the tank above?  Once you know that, count the number of fittings you're going to be using to put it all together.  Let's say a full union ball valve at the pump, the bulkhead (counts as a fitting), the 90 degree at the top, and that's it.  Use a head pressure calculator to determine how much head pressure there will be on the pump.  Here's an example of a head pressure calculator http://www.your-garden-ponds-center.com/head-pressure-calculator.html
Reef Central has a nice one too, but if you don't know what pump you're going to use, then it can be more confusing than helpful (http://www.reefcentral.com/index.php/head-loss-calculator)  So for this example, we have 5 feet vertical height, 1 90 degree fitting, 2 adapters and maybe a total length of 6 feet.  Your head pressure is 7 feet of dynamic head pressure based on that calculator.  There can be a lot more that goes into this, but you just want a rough number for pump sizing. 
4) Select your return pump.  For the scenario that I just described, saying that we have a 120 gallon tank, a Mag 9.5 pump would be a decent pump.  How did I come up with that conclusion?  Look at this document http://www.northlandgardens.com/pages/water_pages/pdf%20sheets/Danner%20mag%20drive%209.5-18.pdf  On the second page of this document, you see the performance curve for the pump.  For a Mag 9.5 at 7 feet of head pressure, that pump is going to deliver about 650 GPH to your display using 1 1/2" return line.  Now, virtually nobody uses plumbing that large...you're probably going to use 3/4" or 1", and so you know you're going to get less than the 650 GPH, maybe not even 600 GPH, but probably pretty close.  Now maybe you hate the Danner line of pumps...you at least have an idea of how many GPH your pump needs to be rated for to be properly sized for your system (between 900 and 1000).  Oh, and to make stuff even more confusing, there's pressure rated pumps, and non-pressure rated pumps...but don't look at pressure rated pumps unless your sump is a fair distance from your display (more than 7 to 10 feet).  Also, over time as stuff gets dirty (covered in bacteria, algae and calcium precipitate), your pump will not perform at peak performance...so just remember that you're basing this on ideal conditions, not the conditions you'll have after 6 months of use and no pump cleaning.  Also, at this point, you may want to go back to the Reef Central head pressure calculator and get a more exact calculation now that you have a better idea of what pump you'll use.
5) Size your return plumbing and overflow box.  Knowing that you're going to do 600 GPH on your 120 gallon tank, you need to know how big the drain line needs to be to handle that amount of water back to your sump, and how large your overflow box needs to be (linear inches around the top edge) to provide good surface skimming.  Use the Reef Central calculator at http://www.reefcentral.com/index.php/drainoverflow-size-calc   This tells us that we need at least a 1" drain line and 9 linear inches on the overflow box.  That means that your box can be in any configuration as long as the surface is in contact with at least 9".  A corner box might be 5" x 5" box giving 10 linear inches (water contact on two sides), but a overflow that has contact on three sides might be more like 2 1/2" on the sides and 4 - 5" on the front...giving 9 to 10 linear inches.
So that's how to properly size your plumbing system for your reef tank.  There's a lot to consider, but your planning will pay off in helping you get it right the first time.  Good luck, and if you're not sure what you're doing, continue to do what you've started doing...ask questions.  That's what the club is here for.  We're here to help.
--AJ

 

Oh, and if you appreciate the help that the club is giving you...support the club and become a member. You'll find that people are more willing to help members than non-members. http://www.greateriowareefsociety.org/JoinGIRS/tabid/94/Default.aspx

--AJ

 

Is there a reason the sump isn't Below the dt?

 

If your sump is sitting on the floor, your return pump is undersized. It's unlikely that you'll be satisfied with the performance based on the specifications of the pump and the configuration of your system.

--AJ

 

Moving to general forum...

 

Have you verified the tank is even able to be drilled? And I agree with AJ on the small pump.

 

It was possible that you had it in another room, behind the room where your display tank was, but elevated off the floor.  I was trying not to make assumptions about how you were configuring your system.  FWIW, here's a link to Marineland's page where they indicate that all panels of their 55 gallon tanks are tempered. 
http://www.marineland.com/sites/Marineland/Documents/Standard%20Aquarium%20Specs.pdf
When it comes to differing opinions, don't trust someone that you're not 200% confident in their knowledge.  I've told you a bunch of stuff about sizing and stuff, but until you understand it for yourself, it's all just theory.  Deciding to take a leap of faith based on what I'm telling you, is a decision that you have to make.  You either feel more confident in what someone else is telling you, than what you know for yourself.  If you're not sure, do the research...find out for yourself online.  Google is your friend.  You'll find lots of differing advice online, but when you bust your tank because they were wrong, don't count on them replacing it for you.
--AJ

 

Here's what Aqueon says...FAQ #1

http://www.aqueonproducts.com/education/faq/

Here, they indicate if the bottom is tempered or not, but not the side panels...which is particularly problematic since the 55s and 75s are really the only sizes that I'm aware of where you have to worry about tempered sides...

http://www.aqueonproducts.com/assets/012/23821.pdf

And as for the pump sizing, did those people know how you planned to have your setup configured and did they have the pump specs in front of them? It's pretty hard to argue with the manufacturer's specs.

--AJ