Bow Valley Tank, Setup Questions

[Cyclo]

Im buying a used tank that was made at bow valley. I want to drill it but I don't know if the bottom pane is tempered or not. Do you guys know if they use tempered for the bottom glass?

Also is 5 holes (6 if the bottom is tempered) in the back too much for 3/8" glass? The tank is 30x24x20. The closed loop will be 3 holes, and 2 return holes from the sump.

Also I want a black background, but also an external overflow box? Is there anyway to acheive this? I'm thinking a black acrylic insert across the back is my only option. My concern is water become trapped between the acrylic and the glass unless it was glued in perfectly.