[untamed]

It is fine to have the holes in the backwall of the tank. You haven't mentioned it, but you need to build an overflow box inside the tank. I think you'll need an overflow box that has top edges that total nearly 2 feet for that kind of flow rate.

As for hole size bigger is better. 1.5" might work, but 2" is better. If you make the first hole 2", you could make the 2nd hole 1.5".

Make two holes. Make 2 holes. You have only this chance to do this right and the best advice we can provide is to make two holes while you have the chance. That or live with the noise and don't come back asking how you can make it quieter....because then we will say...you should have made two holes!!!

Both holes have to be in the same overflow box.

Finally...I should mention that you should drill 2 holes....