Drain plugs were originally galvanized, and were installed on the sealer deck, after the first prime oven. Trunk spatter paint was sprayed at Fisher Body, in the blackout booth just prior to the final color reflow oven. The car was nothing but a painted body shell at that point, with no trim or hardware installed; all the items you mentioned (except the upper shock brackets) were installed after paint in the Fisher Body Trim Shop. The upper shock brackets were installed later by Chevrolet. There's a complete raw stampings-to-finished car article on the 1st-Gen assembly process I authored at the CRG site (
www.camaros.org); just click on the "CRG Research Reports" button at the top of the home page and you'll see it listed - it's LOONG!
