What height did you drill your side overflow

What height did you drill your side overflow?

You want to drill it as close to the top as possible the bulkhead has a rim and the tank has a rim, you need to figure the center of the hole and then get the top rim of the bulkhead as close to the top of the tank as possible.

Keep in mind this is mainly for appearances, (so the water line dosnt fall too far below the trim of the tank)

Also, If I have to 1-1/2" bulkhead for the overflow, do I need to run them through separate pipes to the sump or connecting both to a single 1-1/2" pipe will be able to handle the 480GPH (6X turnover).

Yes they can both connect to one pipe (looks like an exhaust manifold) It would also be wise to connect the bulkheads via rigid tubing (slanted) in the outside rear of the tank. Example

One end of the tank has a 90 deg elbow going down toward the floor.
This would connect to a horizontal 1 1/2" rigid pvc going to the other bulkhead / 90 degree
But the second 90 has a 2 inch pc of pipe that makes the horizontal pvc slanted (for drainage)
Then its a simple matter to Install a gate valve and a breather pipe
So the water has no back pressure and can drain into the sump
As far as how far down the holes should be drilled, that will be dictated by how much of a plastic lip the tank has and how comfortable the glass shop is.
You would want the fitting (outer portion of the bulkhead) rite up to the plastic lip.

Finally, if you have time, please, tell me, the black eggcrate you're talking about, is it made of polystyrene? I just don't want the whole setup to become a "toxic waste" once water will be added...

The real black eggcrate is called a lighting diffuser. Its made to go into a suspended ceiling and have a light fixture on top of it. If you use that there is no problem.