Will this fit my factory mirror housing without any modifications?

Yes — this is designed to mount at factory points using OEM fasteners. No drilling, grinding, or bracket fabrication needed.

Do I need to remove the old mirror glass first, or does the whole mirror come off?

Only the glass lens replaces; the mirror housing stays mounted to the door. Removal procedure varies slightly by body style — refer to your service manual for the safest method.

What should I do if the glass doesn't seat flush after installation?

Double-check that the housing is clean and free of debris at the mounting surface. If the fit is still loose, check that you're using the correct fasteners for your mirror type — older and newer F-150 housings can have slight variations.