![Image](http://www.fordification.com/images/FAQ/70_vs_71-72grille_shells.jpg)
Top: '70 grille shell
Bottom: '71-'72 grille shell
This picture illustrates the differences in the length of the grille insert mounting tabs between the '70 and '71-'72 shells. In addition to the two extra mounting tabs on the '71-'72 shells, the four main tabs on each side are spaced a little differently. The top two are close, but the bottom two are way off.
For future reference, this picture and info has now been posted to the above link. Thanks Mark!