If you're installing a 1.5" bulkhead you'll need a 2 and 3/8" drill (60mm). But confirm that when you buy the bulk head. Different manufacturers make bulk heads in different O.D. sizes. One might need a 60mm hole and the other a 55mm hole. Find out from the people you buy the bulk heads from.
I don't know what kind of overflow box you're going to use. My overflow box is right up against the bottom of the top lip and the teeth are about 1.25" deep in the overflow box.
I have the medium box shown on this page:
Overflow Boxes - English
And this 1.5" bulk head. It's the 5th one down on the left side. You glue a raw piece of 1.5" PVC to the back of it and the inside part inside the tank is female threaded.
Product Listing - BulkheadFittingsEconomy
Into that threaded female side I installed this black strainer. Again, 5th one down on the left.
Product Listing - BulkheadFittingsEconomy
That screws into the front of the bulk head. It just barely fits inside the overflow box. You can screw it in and out while it's all up and running. I do it weekly to clean the strainer.
My hole for the bulk head is about 6" below the top of the tank rim when viewed from the outside. It's about 6" from the side/corner.
These cheap hole saws work great. Shipped in a tiny box from Hong Kong in about 10-14 days.
Diamond coated hole saw
I've done several videos. Do you want to see a video of me drilling a hole in the back of a 29g tank? Do you want to see 3 short videos of me showing the shut down on my tank so you can watch it all drain out and stop without overflowing? They are uploaded to photobucket and I'll link them here if you want to see them.