KISS, is my motto.
DO NOT use those idiotic nylon/steel threadsert things!
If you ever have to remove them and the bolt or insert has rusted, it WILL spin, trust my better (or worse) experience on this!
Dodge used them on the old bar style tow mirrors and without fail at least one will spin and ruin the day, not to mention make a real mess of the door in the end.
Bryan, the screws are 3/4" long X 1/4" and don't use a "deep" sheetmetal thread pitch, use more like a pointed style self tapper (not a drill-style), as that's what Lund used in their kits.
Seal it down with a non-acidic sealer, as silicone is acidic when uncured.
ONLY seal immediately around the screws, using sealer to help hold the part down is totally unnecessary and makes future removal, for whatever reason, much harder.
Mark.