If you have a welder and a way to cut/drill holes into each plate, you could plug weld them together and then weld the sides as well. It would help though if you put a bevel on the plates before welding the sides. I'd cut/drill say 5 holes in each say 3/4 dia. or so and weld them together. You will need to keep one uncut/drilled so you will have a base to weld to. As well you could hard surface it too. Just keep in mind by the time you do all that, you would be well on your way to buying a quality anvil.
Scott