Drill for overflow or closed loop? Sounds like you have it full of water already.
I would. For both.

More than 2 holes.
For the overflow - drill near the upper corners for one of these
http://barraquatic.com/ go to overflows one in each corner. Attach these to the tank using a BH or if you have an acrylic tank, you can glue them to the tank using Weldon #4. Reconmend 2 for redunantcy and for flow.
You can use 2 SCWD's for the closed loop with a pump like a mag 12 or more.
Don't forget to get, union's and ball valves
This is just to get you started thinking.