ometime for a few bucks you can take it to your local glass shop and for beer money some poor sap in the back will give it a shot and your chance of making through tempered glass nill, only suggestion is to keep the water flowing any heat and is gonna pop.
I say drill it at the bottom. Just make sure that the seals are good on the voreflow that you are making. If this is your first attempt at building a tank, then you might want to drill it at the top. Just to be on the safe side.