360 magnum flexplate?

You need to use the stock 5.9 magnum balancer.

You will also need a stock 5.9 magnum flexplate. Not all 5.9 magnums have weighted flexplates! The early years put the balance weight on the torque converter and used a 5.2 style flexplate. So make sure you have a later 5.9 flexplate with the weight. One of the bolt holes won't line up with your converter. It's off by like 1/4" just use a drill bit to "oval" it out.

The 318 motor mount bracket can be made to work pretty easily. Just Google it. The correct 340/360 mount can be tough to find sometimes.

Your 318 exhaust manifolds will work. You will need a new intake manifold with the Magnum style bolt pattern if you want to go carb'ed.

Will you be keeping the Magnum serpentine belt setup? It's an easy thing to do and you end up with a modern high amp alternator. The V belt can be made to work but I think it takes some slight modifications.