magnums have a normal 2-piece rear main. heck might even be the same part number as the LA motors, I've not tried to cross reference because I always just buy a full gasket kit when rebuilding one, and because of the oil pan, stock intake with plenum plate, etc, it's a magnum specific gasket kit for sure (since gasket kits assume stock engine rebuild)

did you confuse "rear main" with "oil pan"? cause the magnum oil pan gasket is one rubber piece all the way around.

all 360s were external balance, all 318s were internal balance. As mentioned, the LA is a different external balance factor than the Magnum, but they're still external.