Will this fill the gap between the bumper and lower valance cleanly?

Yes. This panel is engineered to bridge that opening and align with both bumper and fender. You'll have a continuous surface from bumper to fender with no exposed gaps.

Do I need to prime and paint this before installing?

It accepts primer and paint well, but you can install it first and paint after if you prefer. Test-fit it to confirm alignment, then decide on prep timing based on your workflow.

Does this mount at the same points as the factory filler?

It's designed for factory mounting locations, so you'll use existing holes and attachment points on your truck. No drilling or modification to the bumper or fender required.