Its easier to just get a 70 or 71. While it may seem like the 72-74's are cheaper, they won't be once you're done with you're project! For the Challenger I'm pretty in tune with what you need since I've been gathering parts to do this since I bought my '72. I think everything needed has been covered already, but here goes. I'm converting to a '71, so the inner fenders are pretty much the same. You'll need the headlight brackets though, they get spot welded to the inner fenders. If you have anything other than an early '72 (lucky me!) you'll also need to swap the bumper and brackets. You'll need the entire grille assembly, plus all the headlight and hood trim (or it won't match). You'll also need the latch panel and hood release lever. I'm not sure about the hood latch itself or the hood latch support structure, I've heard they're different, but the one I bought looks almost identical to the one in my car. Then you'll need to convert the side marker lights and tail lights. If you want to fully clone the car, you've only just started. The seats, instrument panel, headliner, and other interior parts are different as well.
The grille alone will set you back well over $500 for just a driver condition piece. Restored versions start around $1,500, and this is assuming you can find them.'70 stuff is relatively easy, '71 is harder as the grilles are often broken up (plastic!). The headlight trim will also cost a fortune, normally over $100 each for driver condition stuff. The upper latch panels are usually in the $100 range, hood latch release levers anywhere from $20 on up. The headlight brackets are another story. Stock pieces are difficult to find at best, they pop up once and awhile (I finally have 2 after a year!). Repro's are available from BE & A Restoration, but they're $317!
You can also get the patch panels from BE&A to convert the side markers from them, $50 a pair. Or you could buy new fenders,which would also solve the bracket problem, but thats $1k! BE&A also sells a complete side marker kit for $425, but you can get driver's condition stuff for half that. Tail panels are around $300 new from AMD, plus shipping.
So what am I getting at?
If you bought new or restored stuff, you're looking at $1k for fenders, $1.5k for a grille, at least $200 for headlight trim (probably more!) $425 for side markers, $300 for a tail panel, $100 for a latch panel...you're at $3,600 already!!! And not even half way there.
I know I already have over $1000 dollars into my conversion, and that only gets me the grille swapped over, with driver quality grille, headlight trim, latch panel, hood latch and support, and headlight buckets. I skirted the hood trim and some quality on the headlight trim though because I spent a small fortune on the '71 only wide trim, which bolts on over the top of the headlight trim and covers the lip, which is usually dented on "driver" pieces. I still haven't pursued the sidemarker stuff, and I may leave the tail panel alone because I'm not trying to clone and I actually like the '72 up tails better.
Especially now, non TA or RT '70 and '71 Challengers can be bought for around $5k or less above a '72 to '74 in similar condition. To clone a car completely over would cost at least that much, and even just doing the exterior would be close. Less hassle to just get a '70 or '71!