Do I need to prime this before painting?

Comes primed already, so you can sand lightly and paint directly, or strip it down if you prefer to lay primer yourself. Either path works.

What fasteners do I need to secure this?

Mounting hardware not confirmed with this part. Verify the factory bolt sizes and head styles before ordering fasteners—most installers use stainless steel bolts matched to the existing holes.

Does this cover fill the entire upper grille opening?

This is the upper side section only, designed to fill the factory grille cutout on the left. It bolts into place and follows the original body line contour.