This topic has been covered several times already, but as someone that has collected about 95% of the parts for the swap (mine will be a 72 to a 71) I'll give you a quick run down.
Headlight buckets- these have to be added to the later fenders, you'll have to find a set somewhere and spot weld them in. BE&A sells plates, but they're not cheap. $317 for the pair, and they won't look stock. But originals are hard to find (trust me, I know)!
http://beaparts.com/list_items.aspx?body=E&cat=E&subcat=37 Its probably easier (although more expensive) to just get a set of 70-71 fenders.
Grille- Not cheap, even in driver condition.
Headlight bezels- hard to find in good condition, used ones usually have dents around the edges.
Hood latch support- Not sure if the later one can be made to work. They look pretty similar, but I haven't tried it. I bought one from an earlier car at a swap meet.
Hood latch- at the very least, the release arm will have to be changed, as it won't be in the right place on the earlier grille, the arms are bent differently for each type of grille. Worst case you may need an earlier latch and may have to move the catch on the hood.
Bumper brackets- for a '73 car you'll need the earlier ones, otherwise the bumper will stick out too far. I dodged this problem with my early '72 car.
Hood trim- The headlight bezels for the earlier cars matches the earlier hood trim, which your car doesn't have. You'll either have to paint the headlight bezels to match your body-color hood trim, or match your hood trim to the headlight bezels.
And if you're looking for the whole "clone" package, you'll also need to change the side marker lights, the tail light panel and tail lights, both bumpers, and quite a few of the interior pieces.
Here's a link to some other threads...
http://www.cuda-challenger.com/cc/index.php?topic=43678.0http://www.cuda-challenger.com/cc/index.php?topic=49721.0[/url
