Definitely on the tank I would've thought!
petrol fumes and all that!
If you are talking about the holes in the valance pressing, e.g. For the bumper's mountings, and the horizontal vents, ZCarDepot in the US sells purpose-made blanking plates for them. They are pre-cut and just need butt-welding in to fill the holes. The kit is only $20. An alternative to replacing the whole valance?
If you are replacing the valance, is it a 240Z one you've ordered? As they have differences that make fitment not entirely straightforward on 260Z/280Z I believe. In fact, those pattern panels even introduce issues for 240Zs, as the cutout for the exhaust exit is in slightly the wrong place (causing me hassle when I fitted my FairladyZ with a finely made Fujitsubo exhaust!).
https://zcardepot.com/catalog/product/view/id/1684/s/bumper-delete-sheet-metal-plates-280z-75-78.html/category/341/