You can do the overflow across the side, most people just dont like to as you'd be able to see into it from the front of the tank.
Two things to consider, usually the overflow when coast to coast does not go all the way to the bottom. It'll stick 4 or 5 inches into the tank and only be about that tall as well at the top, so it really doesn't take up too much room.
Another option is an external overflow box. The back glass is notched for the overflow and flows into the box which is attached to the outside of the tank. This way it doesn't take up any space inside. It shouldn't stick out any further from the wall as plumbing can go straight down instead of making a turn out of the back.
Two things to consider, usually the overflow when coast to coast does not go all the way to the bottom. It'll stick 4 or 5 inches into the tank and only be about that tall as well at the top, so it really doesn't take up too much room.
Another option is an external overflow box. The back glass is notched for the overflow and flows into the box which is attached to the outside of the tank. This way it doesn't take up any space inside. It shouldn't stick out any further from the wall as plumbing can go straight down instead of making a turn out of the back.