In the mid-90's, one of the Mopar Magazines had a great article on repairing a Cuda grille. I followed the instructions in that article and used the same SEM products, mainly the #39508 RIGID/TPO REPAIR MATERIAL. However I don't see that listed on the SEM website anymore, so perhaps it's been replaced by the 39506 Rigid Ure-weld? One of the threads that bullitt posted, lists a different SEM repair material as well. These days there are lot of products on the market to repair plastic parts.
I can't vouch for if the cracks will come back, since my grille has been only briefly in the car since I repaired it. However it's held up for 5 years, sitting in the garage
Here's some before and after pictures.
One thing the magazine article also showed was using some thin aluminum bar stock and using the same plastic repair material, bonding it to the backside of the grille in some of the high stress areas. I did that as well, however I can't seem to find any pictures at the moment. If you're interested I can take some new pictures of that as well.
Travis
72 Cuda