I had a few cracked sections in my 70 GS grille, and fortunately no missing pieces. So i gathered up some materials and took a crack (pun) at it. i first used ABS pipe cement to glue two cracked sections together. It didn't work as well just by itself, it didn't seem to melt the grille plastic enough for the time I had available to press them together by hand. Some clamps prob would have worked better. What worked well was some cut sheet plastic from a hobby store that was used to bridge a crack, using the ABS cement. Next up was some loctite E-120HP epoxy. Worked well, stuck to the plastic like stink and dried very hard. Used it for both a sheet assisted bridge repair and by itself to fix a crack where the parts were not really separated (cracked but still touching due to other supporting pieces). Lastly was some JB weld PlasticWeld and sheet pieces. Worked very well as it dried much quicker than the loctite. It grabbed the grille substrate very well. Doesn't look all that great but is very functional and you can't see it from the front.
Look up "Plastex". I restored a 67GS grille that was near wasted and a few other grilles for a board member along with many other plastic parts. 75 dollar kits was more than worth it. however it always nice to macgyver a repair with whats in the garage already. http://www.v8buick.com/showthread.p...r-honing-my-skills&highlight=67+grille+repair