First off great looking car!
Instead of changing the grille, why not remove the bumper guards and the plastic filler behind the bumper and either find some 70,71 or 72 Challenger bumper brackets?? and get them to tuck in more, it gives them a much nicer and cleaner look. The front of a 72-74 Challenger may not be the best looking Challenger grille, but it does grow on ya, and black is probably the best color for making it blend in better.
Here is my 72 Challenger, notice how the front and rear bumpers tuck in and give it a smoother look.
Good luck.