I seccond what was said above about getting a piece to try it on first. let the drill bit do the work, use ALOT of water - like run the water hose on low, and almost no pressure on the drill. Scott showed me how and the best way seems to be kinda twirl the drill around so the bit cuts in one place at a time, same way you do with a hole saw except this is only a few degrees. you will be able to HEAR the correct sound as a constant grinding instead of a skipping. twirl slowly, maybe 6-10 seconds per circle or movement.
DRILL FROM THE INSIDE OUT. often times the glass will chip right when you finally break through and these chips cant be to big to be covered by the bulk head gasket. When you drill from the inside its a clean hole that is easily covered by the gasket and any chips are then on the outside of the tank so no leaks. Be careful with the edge when you first cut it as it cant be sharp. sand it with sandpaper when your done before installing bulkheads.
internal overflow or external? If external, look into different options for surface skimming when drilled through the back wall. An elbow to a strainer may not give you any surface skimming.
