NOS are still around and not difficult to find if you want to spend a little money:
http://www.ebay.com/itm/67-69-FIREBIRD-TRANSAM-RADIATOR-FAN-SHROUD-NEW-OEM-GM-350-400-engines-/330931358615?pt=Motors_Car_Truck_Parts_Accessories&fits=Year%3A1968%7CModel%3AFirebird%7CSubmodel%3A400&hash=item4d0d0bcb97&vxp=mtr

Remans are available everywhere for $100-ish.

If it were mine, I'd try tackling it with my plastic welder. Do you have one of those by chance? Fun little tool to play with and very effective. Otherwise matting the back side with epoxy-soaked fiberglass cloth will hold forever, just rough it up real good and make sure you're using an epoxy that can take some heat. Some epoxies are removed with heat, no so good for a radiator shroud depending on melting temp. Of course good ol' fiberglass resin would be the other smelly alternative which will take the heat fine.