Magnums swap suggests cutting the U out of the one piece Magnum gasket and using the U from the 360 LA timing cover set. See excerpt below. Is that because the LA front U seals better than the Magnum front U piece? Mine has RTV on the corners and on the U piece, from the builder.
5.9 Magnum “One Piece” Oil Pan Gasket
When using an LA timing cover, it is still possible to use the one piece oil pan gasket from the Magnum engines. Simply use the front ‘U ‘from the timing cover kit—you’ll need one to install the timing cover and water pump anyway–and cut away the corresponding U-shaped piece on the Magnum gasket. A little Rite StuffTM where the rubber gasket meets the new U, and VOILA! Leak free fit!