Well, many things to look at
-73 Challengers never came with a big block
-you have a dependable engine that with some decals, hood script and air cleaner can look like a 340 (if you like)
-318's go pretty good
-440 is heavy and the car will drive different
-who cares how fast you go on the street
-the money to rebuild the 440 and then the money for new brackets, mounts etc.
IF you have a car you are happy with, then why change it. They is no shortage of Challengers with 440's stuffed under thier hoods and I bet you could spend 5k in much better places
-don't get me wrong, 440's are fun and cool to say I have a 440 under my hood, but I don't think you will be happy once you spent the money and installed the 440.
I have a 318 Challenger and had the same questions to answer- I bought a 340 but may still re-install the 318-they get a bad rap but are great engines and are no slouches on the street (they are not a 340 or big block, but can hang their head high)