the 1" thick plate will flex enough to be annoying unless you could through weld, but that would be insane amount of work/rods for the rewards. I've had 1 1/2" plate pinned to a log and it was much less efficient than when I then turned it on it's side and mounted it that way (so was 1 1/2" wide face and 4" tall).
If it's a large enough bit of plate, you could slice it up and weld the pieces together around the outside to make a thicker (say 3 bits of plate), so the force of the blows is inline with the weld planes not at 90 degrees as if you welded to the weight. The thickened lump (eg 3" wide, however long and however high) could then be welded to the weight with good wide fillets and you would get a much better anvil as there would be no flex
Interesting stuff mate cheers. I'm not going to be making any anvils but it's all good knowledge
