If you have all the '73+ disk stuff you can run 11.75" rotors off the later B/R body cars. All you need is the rotor and the caliper brackets, which you can now get from Dr. Diff for $100
here.
Summit sells a nice set of drilled and slotted 11.75 rotors, right
here and left
here.
They work with a 15" rim (and fill it out pretty darn well) and will be a better balance with any of the rear disks that you choose. I run the 11.75" rotors on my Challenger and my Duster, and have the parts to upgrade my Dart's before they hit the road. Easy swap if you have the '73 disks already, as its just the rotors and brackets that you need, you still use the stock calipers (as long as they're the slider type). The larger rotor gives a nice increase in stopping power without spending a ton on new calipers, and they're pretty much the largest set up that will work with a 15" rim.