70 challengers have come down a lot in value. To the point where a 318/383 car can be had for basically the same money as a nice 72-74. Therefore, I don't see the point. Sell your 73 and go buy a 70 if you want it so bad.
15 years ago I really wanted a 70, but even then they were out of my price range. Nowadays I wouldn't trade my 73 for a 70/71 for anything. Partially its the work i have into it, also the look has grown on me.The grill just strikes me as way more badass than the pretty girl 70.
Keep in mind I tossed the side gills, bumper guards, and sucked the bumpers in. Resulting in a pretty clean looking 73. And truth is, the 72-74s were a better built car. Alot of kinks were worked out after a few years of production.
And you can put any motor in them. Stock form they sucked, but by today's standards a stock 440-6 sucks.
Modified is the only way to go.