I had the cap screws on the louvered vents on my '77 280z pullout of the plastic and I still had them bolted in place so I just removed the vent piece and the capscrews and got some epoxy resin and epoxied them back to the plastic. But be careful doing this you don't want to get the epoxy on the threds or you'll never get the nut on or off the capscrew. If you don't have the original capscrews you could use flat head screws in place of the originals (this would give you a large head to cover with epoxy, though you might have to do some grinding to custom fit them). Her's a tip....use some DOOR EASE lubricant stick on the threds and cover the cavity on the hood with it as a release agent that way you can put the vent in place while the epoxy is still "tacky" and place somthing heavy (like a large cinder block and a rag) on the vent to hold it in place while the epoxy "sets" , give it about an hour to be sure. Then you can remove the cinder block or what ever you used and put the nuts on the capscrews to hold the vent in place. Should you ever need or want to rermove the vent all you need to do is take off the nuts and LIGHTLY TAP it free from the under hood side. This worked like a champ for me, I hope it helps you. Good luck and keep Zeeing to it!....Bert Portland, Or.
I used 4 tiny nuts and bolts after drill small holes on the underside of the vent on each end. Since these bolts are on the underside ends of the louvers and through the metal flanges under the hood, they aren't seen.