Sump piping

[e46er]

you will need an overflow box of some kind to cut off a syphon if power goes out and when you shut off the return pump to do a water change

you could drill the bulkhead close to the top of the water level but you will need to make sure you have enough room in the sump to drain the display down probably 2 inches minimum leaving the 20 G sump to hold very little water
FWIW my 40 G sump only holds about 17-18 G of water and some sand and rock

[Aquattro]

You can drill the side and use a 90 as the overflow, but I would suggest at least a small box. Is it drilled already? I can give you lots of PVC, so use your money on a box

[Borderjumper]

I'm actually plumbing my new tank tonight Im using 1" for the overflows, which IMO is good enough as there are two, and 5/8 for the return. I find there's a huge difference in the flow between 1/2" and 5/8.. So I go 5/8".

I don't use overflows. I have elbows with strainers.. It's always worked for me.

[The Grizz]

Do a small center internal overflow box just big enough for 1 -1 1/2" bulkhead for the main drain, choaked back with a gate valve and 1 - 1" bulkhead for an emergency drain, as so known as a Herbie.

That is my suggestion for what it worth.