I pursued converting a '73 to a '71 and while gathering the parts together, two things happened: I started liking the 72-74 styling, and I came to realize even if the swap was done, I'd always want the "real" thing.
Long story short, the '73 got sold (still regret it), and I bought a '71 project.
If you are ever in the northeast Indiana area, I still have all my conversion parts if you still want to convert yours.
'71 steel tail panel
'71 tail panel bezels+challenger emblem (broke in middle)
'71 taillight housings
70-71 quarter skins (I need full quarters for my project)
70-71 rear bumper brackets
'71 parts grill (isn't very nice)
'71 steel panel that goes above the grill and ties the fenders together
You would still need:
'71 headlight bezels
70-71 front fenders
70-71 side marker housings+lenses
70-71 front bumper brackets
+any of the other junk I don't know about
If I were in your shoes, start with changing all the bumper brackets to tuck them in like a '72 car, the styling might start growing on ya, good luck with the project!