I've been cleaning on my fan guard and it is not in to bad of shape. However there is a crack and kind of dull. How would be the best way to repair the crack and should I use a paint on it?

I attempted a couple different epoxies and cements to fix a crack in mine and none worked well. I ended up getting a new one and GM original parts were still available and may still be.

ArmorAll works fairly good to freshen up material like that if you can find a way to fix the cracks.

I repaired a crack in mine with fibeglass cloth and resin on th inside. Seems to be working well after 3 years...

Black shoe polish might do the trick. Works on the "rubber" step on my stepside pickup. Lasts a good while. I would try it on the bottom of that part first...or plastic paint.

Epoxy should work. Find a piece of material to bridge the gap by about 3/4" on each side of the break, clean with a mild solvent and soapy water, abrade the area with fine sandpaper, when you have everything laid out and ready to glue take a propane torch and quickly flame treat the repair area, not enough to heat the plastic just 2 seconds or so. then glue and clamp the pieces together.

mine had a piece missing, bought a new GM one...think it was $59 8 or 9 yrs ago.

They can repair plastic bumpers on new cars. Perhaps the same product will work on the guard?

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.

Have you tried superglue? I have used that on a couple of parts now and it seems to work great! Very simplistic.

Thanks for all the ideas! I'll have to see how I want to tackle it.

+1...put some resin on the underside, lay your cloth, more resin, smooth it out, works like a champ.