I've never heard of a 40 breeder being tempered anywhere but the bottom, and I've drilled at least six of them for other folks in the past- all Aqueon from the sales. The reports I hear of them being tempered all panes are odd, and I'm not sure if I trust them as fact.
Make a dam out of putty to hold water around the location to be drilled, keep a running hose on the hole as you drill, and take it slow. I've only ever had minor chipout when drilling, and that can be prevented a bit by holding the back of the hole with masking tape or putty, that way the waste piece of glass doesn't start to fall out unevenly from its' own weight. It will be messy, and you'll be a nervous wreck your first time (take it slow, baby ) but I think drilling your own is a fun experience
Make a dam out of putty to hold water around the location to be drilled, keep a running hose on the hole as you drill, and take it slow. I've only ever had minor chipout when drilling, and that can be prevented a bit by holding the back of the hole with masking tape or putty, that way the waste piece of glass doesn't start to fall out unevenly from its' own weight. It will be messy, and you'll be a nervous wreck your first time (take it slow, baby ) but I think drilling your own is a fun experience