La timing cover and Balencer

If you have a Magnum engine with stock crank, pistons, and rods, you need the Magnum balancer to properly match your engine. The balancer gtsdude suggested works well if you don't already have the original for your Magnum engine, or if your are switching from the newer one-piece serpentine pulley arrangement that started some time in late 1999 production.
The professional products balancer has bolt-on weghts that allow you to match whichever 360 LA external, 5.9 Magnum external, or even no weights installed for internal balance, whichever your engine requires.
Finally, if you have the stock Magnum balancer that matches your engine, and does not have the integrated serpentine pully, you can install your old LA v-belt crank pulley by slotting or elongating the one bolt hole that does not line up.