I've used cop car gear boxes on both my challengers and love them. I got them out of mid-80's diplomat and gran fury cop cars. The ones I got were in good shape and come with trans fluid cooler too. I just swapped a new pitman arm on and they worked very well. I love the firmer steering, much more like a modern car. Each box cost about $35 at a junkyard, but pulling them is a real pain. You have to pull the exhaust manifold and cut off the exhaust pipe to get it out. I think I had to remove the battery box too.
If you have a small block, those cop cars have great kickdown linkage too. I'm told they all have the 11.75" discs up front too. They also have much more stout transmissions. Lot of great parts on a cop cars.
Don't overlook rebuilding the steering coupler too, they need to be packed with fresh grease. I think year one, hardens or the paddock have those parts new. It can really take the slop out of the steering if they are worn.
