Definition of tempered glass found on google: "Treated glass that is strengthened by reheating it to just below the melting point and then suddenly cooling it. When shattered, it breaks into small pieces. Approximately five times stronger than standard annealed glass; is required as safety glazing in patio doors, entrance doors, side lights, and other hazardous locations. It cannot be recut after tempering."
It can not be drilled either. It is mainly used on the bottoms of tanks, so most likely the sides are standard glass and the bottom is tempered. Thats still ok in your case because you can drill the hold in the side glass and still put a bulkhead on it.
As for an overflow system, Yes you can buy overflow kits but If it was me, I woudl DIY it because I cant see spending money on sometning i could make myself. Its basically just a piece of Glass/acrylic that you silicone into the corner of your tank. It will have "teeth" cut in the top of it where water can "overflow". the hole you drill in the glass will be behind this piece of glass/acrylic and will drain to your sump. The advantage, as was stated above, is that the water level in your tank will never drop below the teeth in that piece of glass/plexi, and there are no syphons to worry about failing/starting.
I know there are many threads on here about DIY overflows. Here's one with the durso stand pipe too....
https://www.saltwaterfish.com/vb/show...light=overflow The durso is not necessary and i dont have an overflow right now so i dont have one, but people say that they are much quieter than just a pipe in the overflow.
