The problem with putting a '70, '71, or even a '71 Barracuda grille in a '72 Challenger is that the '72 fenders don't have the headlight brackets like the '70/'71 fenders do. On the '72, the headlights bolt into the header panel, which makes up the headlight brackets, grille opening, hood latch mounting panel, etc. Once you remove the header panel from the '72, you don't have any way to mount the headlights, hood latch, hood release, etc. You've just got a big open hole.
The "easiest" way to do the conversion is with a set of '70 or '71 Challenger fenders. Bolt those on and you can mount the headlights again. But you still have to track down a hood latch panel, hood latch release, headlight bezels and a grille. And, just for fun, all of those parts are one year only, either '70 or '71.
You can also order a set of headlight brackets from B/E & A parts, they sell a pair of headlight brackets that you can add to the later fenders. They're pretty expensive, but then again a set of earlier fenders would be too. And of course there would be nothing stock about their installation, although you wouldn't see them once the headlights, bezels, and grille is installed anyway. http://beaparts.com/list_items.aspx?body=E&cat=E&subcat=37
It's not a short list of parts (I've probably forgot a couple), and of course anything related to the '70 or '71 cars is expensive. The '70 grilles are cheaper, they made more of them and they're metal so they hold up better. The '71 Challenger grilles are plastic, so finding one in good shape is difficult, and should you actually find a decent one you can expect to pay a TON for it. How do I know? I have a '71 grille for my '72 Challenger. And a set of fenders, and a hood latch panel, release, headlight bezels etc. to clone my '72 over to a '71. Yes it can be done, no it isn't cheap, and it actually took me a few years to track down all the parts. Had I tossed my budget I probably could have done it faster, in some cases I waited to find more reasonably priced stuff instead of the first thing that popped up.
The Barracuda grille would be another headache. The one on the "chacuda" seems to fit pretty well, and it would make sense that the overall dimensions would be close. But, the devil is in the details- like how the headlight bezels interface into the fenders, how the bumper lines up, where the hood latch goes, how the valance gets installed, that sort of thing. And since the "chacuda" has frenched in bumpers, shaved marker lights, doorhandles etc there has been extensive bodywork done to that car. Maybe it only takes a few custom brackets, or maybe it took reworking the entire front end. It doesn't look like anything really
crazy had to happen, the hood and fenders still have most of their original landmarks, but those pictures are pretty small and it's hard to tell unless you have a stock example sitting next to it. And until you had a '71 Barracuda grille in your hands, you wouldn't know what you'd need to do. And even a reproduction '71 Barracuda grille will set you back about $800
Here's a video walkaround of the "chacuda". Based on the hood open shot it looks to have a '70 or '71 Challenger hood latch panel in it, ditto on the hood latch release. https://youtu.be/Mt0e4kCI5wk