If you are serious about trying (and are sure its not tempured) try going to this web site:
http://www.glass-holes.com/
They sell glass hole cutters and overflow kits for aquariums and they have a bunch of how-to videos that show you how.
I drilled 3 holes for 1" bulkheads in my 75 gal. sump without much drama (I don't think I would have had the guts to drill my main tank however, but that's just me).
I order 2 hole saws, great service and quick.
Good luck.