The ever returning problem with fitting new speakers into old cars...
If you look hard and long enough, you
should be able to find something that fits, but you may have to fabricate some brackets. Heck, I even managed to fit two decent Pioneer speakers into the single mono speaker space in the dash of my '63 Thunderbird convertible. (I admit: no Mopar, but there is not a single Ford-emblem on that car either...
) Unfortunately, the room for the radio/tape deck was very slim, and I do not have the stomach to cut up that dash. So the quality of sound still leaves a lot to be desired; I am not happy with my system from Custom Autosound...
For some ideas regarding Challengers, have a look at this:
http://our70challenger.moparpages.com/html/sound_system.html They did have to cut the dash a bit for their outside speakers, so maybe this was no help at all?
An excellent site that might give other good ideas, anyway.