How can I build shelves that are 66" wide?
You can put a 1/2"-thick wooden board over the metal piece to camouflage it. And if the shelf is really that long, you'll need something strong for the metal piece. A structural engineer could probably tell you the minimum dimensions, but I would guess a 1.5" angle, or square tube, would do it. And that means you're limiting access to the shelf below that.

If this was my shelf, I'd focus on finding shelf supports that take up as little space as possible. Something like this: