The original firewall insulator was a pressed board cover over a jute composition mat against the firewall. Insulator retainers were pushed through holes in the cover and matting and stuck out the firewall through matching holes. The retainers are broad flat head with a folded metal tongue which has teeth cut on both sides which grip the fire wall holes and hold the insulator in place.
Since you have opted for a hole-free firewall I guess you can use some heavy duty 3M spray adhesive to hold up the mat, then spray the reproudction fiberglass insulator board and stick it to the pad. The mounting screws for accelerator and other under dash components (a/C bracketry etc) should hold the new fiberglass board in place with no trouble.