On Magnum blocks that are factory drilled for LA oiling, you need to make sure the two camshaft bearings are drilled or lined up with the oil holes in the block. Equally important, you must run a camshaft that is drilled for LA oiling as well.
If your block is drilled, camshaft bearings are drilled and lined up, and LA head gaskets are used with oil holes, everything else can be regular LA heads and rockers.
If block was not drilled for LA oiling, use the Magnum lifters and hollow pushrods along with LA rocker arms designed for pushrod oiling.