I just picked up a used 40 breeder. I don't know the manufacturer, there is only a white tag with a date that is partially under the bottom rim, and partially out.

I definitely want to drill it. Any suggestions on which spots to drill? Can I drill the bottom?

It's probably ok to drill the bottom. I don't think AGAs are tempered and that's probably what you have.

didnt drill the bottom on mine for the same reason... little white tag worn away and unreadable. Just drilled the back

I drilled the bottom in my 40B. No problems there.

If its older you should be fine. Even now I know AGA/Aqueon only temper the bottoms of their 50 breeders

I drilled the bottom of a couple of 40B's, none of the mfg's temper the bottoms of that tank.