Steve, that will work just fine. You will occasionally see "cutlers" anvils here and there, they were just square pieces of steel, no horn, no tail, no hardy etc... and they work great. I have a 5 x 5 x 12" anvil that I use alot. In some ways if you are forging blades the "square" anvils work better as you can get good hammer clearance by working each "end" of the anvil face.
One thing I would suggest is once you figure out how you are going to set it up, dress/radius the edges real good. Its also a bit tricky to fabricate a base that wont/dont want to move under heavy hammering. If you make up a wood base, which is easy to do with 2 x 6 & 2 x 12's, put a thin peice of rubber or carpet between the concrete and wood, it helps alot. You would think there would be alot of energy loss, but there isnt, not in my exeperience anyway.
Good luck with it, post a follow up when you build your base.