Have not tried the classic Challengers, but on my other Mopars from that era, push in on the bottom seat about a third of the way in and pull up. Do it to both sides and pull out the seat bottom. Your build sheet should be under one of the springs providing someone hasn't removed it yet!
BTW, won't you go to the Newbie Section and introduce yourself and your Challenger for a warm and rousing Z Welcome! Thanks!