I purchased the lower valance from Firebird Central. Eight bolts hold it on and the bolt holes are already in your car, though the threaded u-nuts may not be. This piece is not pre-drilled. You will need to drill the mounting holes in the valance and fit it to your car.
Anyhow, I didn't use the piece I received. It would have worked, but I didn't like the way it fit. What I did instead was use it as a template and made my own from 4" PVC schedule 40 pipe. It took awhile flattening out the pipe, but after that things went quickly. The panel itself is flat and 1/4" solid PVC. It's going to do the job of closing out the radiator. I also installed the upper closeout panels too.
Back to the store bought fiberglass panel though. I think it would be better to sell these oversized and the buyer has to trim to fit. Back in the day our cars were pretty much the same size, but after 50+ years dimensions change here and there.
You made the template Why not just make it out of sheet metal ?
I don't have any sheet metal - steel or aluminum - and I checked prices on a sheet big enough and it was too expensive. I had the PVC pipe sitting around for ages so that material was free. In the end solid 1/4" PVC is as good (possibly better) than sheet metal or sheet aluminum. It's easy to work with. It's lighter. It won't rust. It's more than strong enough for its intended purpose. I'm not hung up on everything has to be metal.
If anyone is interested I could dimension my template and you could make your own close out panel. I don't recommend buying the fiberglass part I bought. Maybe it'll fit perfect on your car, it didn't on mine. You can make this from any type of stiff sheet goods that is weatherproof. I used PVC pipe. You can get 1/4" PVC sheets from building suppliers, though this isn't solid PVC. It's cellulose PVC. It will probably hold up just fine. The 4x8 sheet will make 4 or 5 close out panels. Heck, 1/4" plywood would work just fine too. Fiberglass, steel, and aluminum are choices too, if you have access to a cheap supply. This part isn't structural. I wouldn't spend very much on materials to make one of these. There's no need to.