Never heard of using body filler for this job but here's a link to another method using it. No idea if it works.

http://www.automedia.com/Plastic_Grille_Repair/res20011101gp/1



69 EF8 GTX 440 auto
72 EB5 RR/GTX 440 auto
70 EW1 GTX 440-6 4 spd
70 EF8 Satellite 318 auto (452 coming soon)
70 FK5 Satellite 318 auto
71 FC7 Cuda 340 4 spd