This is a tough problem because the anvil holder is the one assembly that must be accurate and "true". It holds the anvil axles and if the base is fabricated unevenly, then you have an uneven pressure problem when wheeling sheet metal. To address this issue I decided to mill the two anvil holder sides with one milling operation so that they will be mirror images of each other.
To start the process I used some scarp 3/8" plate and milled the edges square using a 5/8"-4 Flute end mill.
We'll never forget !!!
Next the bottom gets the same treatment. When doing this process don't forget to lock the quill, or the spindle may bounce and give an awful finish.
This looks simple, but there are some "gotcha's". I make sure that the vise is square to the mill head, and that the "X" axis table lock is tight so that the table doesn't start wandering in a lateral manner.
The sides are milled square with the plate set flat in the vise. This time the "Y" axis jib locks on the side of the mill table need to be tightened so that the table won't move. Remember that vibration alone can move the table.
Done!! Milled on all edges, so now it's nice and square.