It is probably not all that hard to HT an anvil.....holding a 280# piece of red hot steel in the tongs for quench is a bit of a trick, though. I saw a video clip a few years back of a fellow who built a big fire on the lake shore and heated up his anvil for hours. He had a big eye bolt through the hardie. He hooked it with a chain and swung it into the lake.
I have a 300# post anvil that I made from 7" round. It is unhardened. I reground the surface recently for the first time. It will work harden a bit with use.
Figure the working height you want, and cut the piece to length. ( I find 30" does well for a post anvil. Save the other piece as a bench anvil.) Weld the post steel to a 24" round of 1/2" mild steel. To move the anvil, tip it at an angle and roll it into place. (Hint - round the edges of the steel disc a bit, to prevent it marring the concrete floor)
Stacy