I have a perfect rust free 67 to 72 K memberThe reason there’s no good compiled source for a magnum swap is because it’s not needed. Externally, it’s an LA small block with a serpentine setup and fuel injection.
Just look for a 68-76 small block K-frame. Keep an eye on the for sale sections here, there’s good deals to be had. I wouldn’t worry about the rear-end yet, a stock small block won’t destroy it immediately if you’re kind to it.