While you're swapping the manifold, do yourself a favor and install Hughes Engines' manifold plate kit. It solves once and for all the annoying habit Magnums have of blowing out the plenum plate gasket.
https://hughesengines.com/Index/products.php?partid=27091
The kit also includes new manifold gaskets, bolts and other hardware so you won't need to source them elsewhere. Really, it has about all you'll need to swap everything over between the two long blocks, and fix the plenum issue to boot.
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