I have read good reports about using drums of sand as anvil bases. Not seen anyone doing this first hand, yet.
55 gal. drum might be a burden to work around due to its diameter, depending on anvil size. Thinking all thats needed is something big enough to get the base in without touching the sides and enough footprint to be stable.
Think I would look at something like a 30 gal. drum, they are also available in heavy wall thickness like the big ones and would hold enough sand to keep the thing from walking around. But then, my anvil is not 500 lbs.