Take a look at some other anvil stands and use that as a fabrication design, you want the legs to be angled out some for stability, rectangle tube 2 inch by 4 inch is a good choice for the legs
I know this would require a complete rebuild at this time, I would try to find a piece of heavy plate for the anvil to sit on 1 inch plate would be good, then weld the legs to this for a tripod stand. You could save the other stand and use it for a saw horse. I like to look at what other people are doing when I start a project and take that information into the design stage, find out what others are doing, if it works well I will use it