Well that makes me seriously rethink everything anyone has told me about glasscages.com. How can they make such large tanks without tempered glass!? Your glass would have to be really thick on that 180. They are correct that you can not cut or drill tempered glass. That does not mean you can not customize it. All cutting and drilling just has to be done before the glass is tempered. Generally on most tanks only the bottom pane is tempered. The surface of tempered glass is free of all imperfections which makes it extremely hard for cracks or faults to be in the glass. The surface is actually under a tension because of this. That is why the whole pain fails when a crack or cut is made. The edges of tempered glass are very week, a slight wack on the edge can cause the whole pane to fail. This is why it is not normally used on the side panes of a aquarium that have exposed edges.
Any 55gal made somewhat recently will do this
if you try to drill the bottom. This
video does a strength test between regular and tempered glass and reexplains why tempered is so much stronger.
So if I was in your position I would only use that tank if your repair of the bottom pane uses tempered glass. Tempered glass is not cheap, especially custom, your best bet is to ask at local glass shops who may be able to make one for you. Google shows a lot of people complain about their tanks cracking and apparently crappy seal jobs.
For those interested. Next time you are at petsmart look in the larger tanks, usually their is a hidden sicker under the bottom trim visible only when the tank is totally empty, looking down in to it. Says "Tempered glass".