Will these mount using my existing mirror holes?

Yes — these are designed to use factory mounting points on your Super Duty door. Existing hardware holes align with OEM locations.

Do these mirrors stay stable at highway speeds when extended?

Manual mirrors hold their position once adjusted. Test your adjustment before hauling; mirror vibration at speed depends on how firmly you lock the adjustment mechanism in place.

What fasteners or hardware come with these?

Mirrors ship as complete assemblies ready to bolt to your door. Confirm mounting hardware compatibility with your truck's existing setup before installation.