I would take the heaviest piece and cut it off anvil length and taper one end for a face. Then, 2 of the net heaviest pieces for an base and spacer near the length of the face piece. Weigh the pieces. If not over 100# make an identical top piece and if necessary bottom piece, even a a second spacer piece. I would then weld the spacer(s) perpendicular to the base and then weld that to the face. I would uses very heavy welds. When welding on the face I would take my time so that the face never gets to hot. You could set the face n a shallow tub of water to keep it from over heating. If I used a doubled pieces I would grind a bevel the edges that would be welded and fill weld the resulting grooves. Clamp together and then put, a heavy tack on each end then a couple on each side. I would move around a bit as I welded to keep the weld stresses even. It would be best to preheat the pieces to around 450 before welding. Id I used a double spacer I would grind the top and bottom welds so that they were flat before welding to face and base pieces. If a double top I would grind the joining weld smooth. If the steel is about 3" wide and 2" thick and you doubled everything you would come out with an anvil in the 150# range. Unless they have to move it alot, I have never head of someone complaining their anvil was too heavy.
If your real ambitious you could lay out a 1" square towards the end of the face piece. then drill four 1/4" holes just inside the square, then drill a 1" hole in the center and finish it off with a die grinder and or files for a hardy hole. If doubling the top, just drill the bottom piece oversized in line with the face plate hole.