The LA timing cover has two one inch long extensions just before the timing cover gets to the pan rail. The Magnum timing covers do not have those, so the pan either needs to have them flattened or fill the voids with silicon etc. Consequently, the Magnum pan wont fit over those LA extensions without modification to clear them.

As mentioned above, its easier to use an LA pan on a Magnum, but you have to fill the 2 voids in some manner.

Here are a couple pictures of my LA pan and the one piece Magnum gasket & the 4 piece LA gasket. Notice how the blue Magnum gasket effectively hides the 2 gaps with the flaps of the gasket. It took me a while to figure that out.