This is a good question and one that many struggle with when restoring their cars. If you plan to have your car acid dipped or media blasted, as William pointed out, this will destroy the tag so you should remove it.
Some body shops recommend that the tag be removed to protect it and assure it is kept safe with the owner until the shop is ready for it. My body shop media blasted the body shell and wanted to make sure the surface under the tag was clean, rust free, sealed and primed as the rest of the body before painting. You donít that too much primer and paint build up around the edges of the tag.
Photo document your tag and try to get pictures of the rivets from the back side, in the cowl well area, using a small digital camera. You will see how the backside of your original rivets flair outward. Use a small machine screw that will fit thru the rivet and a nut, with washers on both the front and back side of the rivet, to secure the rivet.
After my car was primed and ready for paint, I instructed my body shop to then install the tag and paint over it with the firewall black and body color overspray as close to original as possible. Provide the body shop photos of your original tag showing the overspray paint. By the way, also document the black/body color feathered edge along the top edge of your firewall, becuase it was not a neat, masked straight edge. One last detail - remember that the rivet holes were filled with seam sealer before body paint, as shown in your pictures.
I purchased NOS rivets. They are also available from AMK Products, www.amkproducts.com
, 540-662-7820. You can view a PDF copy of their catalog on-line. Look for rivet item #B-13182 listed for Ford and GM Data Plates, $2 for two rivets. Buy a set of spares.
Attached is my reattached trim tag. You can judge for yourself if it looks correct. View others on the CRG web-site, and check out others at car shows this summer.
Good luck with your project!