It is a hard topic to find but this is what I found.
http://www.edmunds.com/insideline/do...ticleId=109513
http://www.allpar.com/model/challenger.html
https://dodgechallenger.com/history/...er-history.asp
"1973 brought about almost no cosmetic change from the 1972 model year Challengers. One slight alteration was the addition of bumperettes on the front bumpers. The grille section, and fender gills remained the same. The Rallye option was offered, but only as an option package on the base model, and Dodge offered a sort of "build your own car" program, where you could piece together the various offered options in order to make the car exactly as the buyer wanted it, and evidently, even with the same balmy engine options as in 1972, sales increased to 32,596. Midway through the 1973 model year there were rumors that Dodge would drop the 340 cubic inch engine to make way for the new 360 cubic inch engine, which boasted a whopping 5 horsepower increase over the 340, but this engine would not debut until 1974. Despite the 240 horsepower engine, the 318 cubic inch engine was by far the most popularly chosen engine in 1973."
This is something that is hard to find. I wish you luck if you want it anymore specific, because this is as good as I was able to find it.