Magnum serpentine
When I did the magnum head swap onto my LA engine I had heard the accessory holes were in different places. With an LA head and magnun head next to each other on my bench and measuring with a straight edge I could not measure any diffference in the locations of the accessory mounts on the heads. My LA accessories bolted right up to the magnum heads with no modifications. Also, several buddies of mine have purchased 360 magnum crate motors and had no issues bolting up LA accessories.
So, no need to change the heads.
You will need to swap the timing cover and water pump plus get a magnum fan since the serpentine system turns the water pump in the opposite direction. Since the magnum cover has no provisions for a fuel pump you will need to use an electric pump.