It's just like putting Lipstick on a Pig!
It just seems wrong to make a '72 a '70. It's a creation (or abortion!) of something it's not.
So, that would be about the same as taking one of the TEN THOUSAND slant 6 Challengers they built in 1970 and putting a V8 in it right? Or maybe like putting a big block in a small block car? Gosh, what about all those jerks with '70 Challengers that add R/T or T/A badges! Making it something it's not? They built more Challengers in 1970 than they did in 1972, 73, and 74 COMBINED. Which ones are rare?
The swap is pretty involved to do it right, even just on a cosmetic level.
The headlight buckets are the biggest issue, since the '72 fenders don't have buckets. Its easier just to get '70/'71 fenders, otherwise you'll have to find buckets. The fiberglass ones are a disaster with metal fenders, as you'll have to use screws or rivets to attach them. Not good.
The grille header panel also has to change, as well as the hood latch support structure, and the hood latch release, since they're designed to work with their corresponding grille.
Then you need the grille itself and the bezels. Also, remember that a '72 has painted hood trim, not chrome, so it won't match the bezels. It also mounts differently to the hood if I remember correctly, so its not quite as easy as buying '70 hood trim and bolting it to the '72 hood. The trim on the front valance is also different.
You can get metal headlight brackets and side marker patches from B/E&A, but I don't know how hard it would be to get them to ship to you where you're at. And they aren't cheap! They want $317 for the buckets. Side marker patches are $49, a little easier to deal with. http://beaparts.com/default.asp
Also, if your buddy is interested, I have an original set of '70/'71 headlight buckets for a Challenger. I was going to use them to clone my '72 to a '71, but I found a set of fenders instead. Shipping would probably be a pain though. PM me if you're interested though, maybe we can figure something out. They would spot weld into the '72 fenders just like they did on the '70/'71 fenders.
Then of course there's the tail lights, which requires a whole different tail panel, not to mention the entire tail light assembly. There's a TON of other differences, but just to make the outside look like a '70 is pretty involved. The list of parts to clone my '72 to a '71 so far looks like this
'70/'71 front fenders
'71 headlight bezels
'71 header panel
'71 hood latch support
'71 hood latch release
'70/'71 hood with latch
'70/'71 side marker quarter panel patches
'71 Rallye gauges with 8k tach
'71 only wide hood trim (no, I didn't need it to make it a '71, but it does look nice!)
I'm sure I'll find some other things when I try to mount that grile in there. Some of my list was a little easier to rationalize- my original fenders have rust in the dog legs, so buying a set of good condition '70/'71 fenders actually made my bodywork easier.
And I'm not swapping the tail panel, because I like the '72-'74 tail lights better. Plus, since my '72 was actually built in 1971 and therefore has a 1971 build date, I want something on the car to keep it from being passed off as a real 1971.