you can drill anywhere on the non tempered pane. The location will depend on what type of plumbing you plan to use. I know a lot of people use kits from Glass Holes. I think they had issues with RC so if that shows up as ****s then pm me. You can find them by searching google for their name.
One thing to be careful of - if this is a 55, many 55s have fully tempered sides and bottom. So just make sure that the side you are drilling is not tempered. The sticker normally states something that makes it obvious that all the glass is tempered on the ones I have seen.