On Moparts they recently shared a new product that may be helpful. Moser has a solid axle swap kit for new generation Chargers and Challenger. Should work for the Magnum as well. However, I don't know if you can get it in a narrowed configuration so you can tub the car. Here's the link: https://moparconnectionmagazine.com/moser-engineerings-solid-axle-swap-kit-for-2009-up-challengers/

There is also a company that sells a "live axle" replacement module that bolts in place of the factory independent rear suspension module. I doubt that can be narrowed, but just thought I would share it: https://acmehpl.com/store/ACME-Big-Stick-Traction-Enhancement-Module-p203760486