the LA rocker gets its lubrication trough the shaft from a passage in the deck of the block. This is connected to the cam bearings. It´s a pretty good system. Without pressurized shaft oiling the LA rockers will get in trouble, sooner or later.
the Magnum rockers are setup to get oil trough the pushrods, no pressurized shaft here. No passage in the block to the deck, thus no chance to put LA heads on the Mag block without machine work....the passage can be drilled, but i do not know if it´s cost effective......The other way is possible, mag heads on LA Block, no problem with the right parts.
Many aftermarket lifters are made for pushrod oiling, with hollow pushrods there will get some oil to the rockers in an LA-setup.....but i doubt that´s enough to do the job. Some guys use both systems, shaft oiling and pushrod oiling, to improve valvetrain lubrication. But as told before, i have no idea how your heads are setup to accept both oiling systems.
Michael