How do you calculate the safe working load of chains?
Another thing to consider is that the effective load on the chains is increased if the chains are not vertical, as would be the case if the chains are attached to the corners of the item, but are all attached to a single central attachment point above. Multiply the chain's working load limit by the cosine of the angle between the chain and vertical to get the adjusted limit. For example, if 1000 lb rated chains are at 45 degrees to the vertical their safe working load limit is reduced to 707 lbs.
For overhead lifting where human safety is involved you should use Grade 80 or above. You would probably have to go to a specialty dealer to find it. Proof Coil, Grade 43 (Hi-Test) and Grade 70 (Transport Grade) are not rated for overhead lifting.