Kong's got a good suggestion there.
I'm not sure how you'll be able to deal with the inevitable warp of the plate, no matter what you do, it will end up convex on the face, and should be milled into square and flatness. I'd opt for something thicker, to reduce the warp factor and also to give a little extra material for facing.
Or, if you have access to a large enough lathe, you should be able to center drill the plate, and then face it between chuck and tailstock after welding.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)