The final width of the housing/axle flanges depends on the offset of the wheel you plan to use. Mark or note the side to side pinion position, it will be on the right side of the cars centerline. Then I would suggest you remove the old rear end, and mock up the actual slicks and wheels in place. Make sure you allow for clearance when the suspension compresses. With the tires and wheels in the desired location, and the car at the desired ride height, measure the distance between the wheel mounting flanges. Then you know exactly the correct width you need.
When you narrow the new rear end, make sure you allow for the proper pinion offset. Most MOPAR's have the pinion 1-3/8 to 1-3/4 inches to the right.