Aftermarket front and rear bars are available for Challengers (e.g., from Performance Suspension Technology). A front bar is at the top of my mod list as my 72 did not come with one.
I read, I think in my manual, that rear bars were only an option on certain Challengers; I read somewhere else that they are not necessarily a good idea. A couple of years ago I called the Mopar Performance tech line to ask for more info on rear bars and they said they would not recommend them - they even seemed reluctant to discuss them. Would they tend to make the rear of the car too likely to break loose? And would this be affected by size and power of engine, rear springs, torsion bars, etc.?
I would be satisfied with the front bar at this point but it would be interesting to know how much handling both bars could give you.