Magnum serpentine belt on a LA?

Short answer: No.
Long answer: Anything's possible.
As mentioned above, the entire timing cover/water pump assembly needs to be swapped from the Magnum- It has specific mounting points for the serpentine brackets. This will bring up bypass hose alignment issues with the 318 manifold, and you lose the ability to run a mechanical fuel pump. I'm not even sure the Magnum timing cover has enough clearance to run the long-nose LA cam; I never tried it. Use an early Magnum (bolt-on type) crank pulley on your 318 balancer. Your 318 also has a centered thermostat housing on the intake manifold which will interfere with the bracket; you will need to remove sections of the bracket, or switch to a late '70s (not sure of the exact year of the change) type manifold with the smaller, offset thermostat. This may not be an issue if you're not running AC, the offending section of the bracket can then be removed. There may still be issues with clearance to the LA heater hose nipple location. I don't believe all the mounting bosses are all there on the LA for mounting the belt tensioner and PS pump.
Can it be done? Sure. Is it worth the time/effort/frustration? It's not a bolt on. Better off getting one of the aftermarket serpentine setups if that's all you're after.