You could use nuts and bolts with fender washers. Tractor Supply has them at much lower cost than the box stores.
I have the pins but think I will push a long pan head machine head screw from the engine compartment into the passenger compartment, then secure to the pad with a fender washer, lock washer and nut. It will take two people.
I used spray adhesive on one, glued it perfectly in place...however I hope I never need to remove it! I’ve also used plastic ‘Christmas tree’ clips from Sherwin Williams. They pushed in and worked fine, even sealed the holes for the most part and didn’t scratch paint.
I'm currently installing the firewall pad in my 65 Fastback. Behind the OEM-type pad that you can see, I also have sound deadening layers of Mass Loaded Vinyl (MLV) and Closed Cell Foam (CCF).
I used the machine-screws and fender washers as "bmcgc" suggested. The long screws initially made it easier to align and hang the pads on the firewall. After everything is properly fitted, I will replace the long screws with shorter ones.
I am doing the installation solo, but I agree it would be easier with two people.
I used 3M spray adhesive - one coat on the pad and one coat on the wall. Let it dry, then make sure you get it in the right spot. Most difficult thing is trying to cut the pad. I laid what was left of my old one over the new one to mark where the holes should be. Then cut the holes... yeesh!
My son and I used some plastic push tabs on his that looked a lot like the metal originals. They worked great. I'd rather not glue one in case I ever had to tear it off. Besides, I like that original look, even under the dash and poking thru the firewall underhood.
You an find these in a Grainger catalog: Insulation pins. About 4" long with a 2" X2" plate on the bottom. You glue those to the firewall. Push your insulation pad over the pins. Secure with the clips that push on also. (those should be in the catalog too.) The clip the excess length of the pin off.