So I do have a question for you my friend. Why did you go the extra mile as you did? I don't mean it sarcastically at all.
I flatten slabs with scrap wood and they come out perfectly flat. Joint a couple 2x4's, clamp them to my flattened work top, and shims to balance out the twist. Once one side is done there is no need to shim the other side, just a jack plane to take off the fuzzies. no doubt your work station gets it all done and is smooth to use, but did you see a great need to make such a piece? Or just wanted a dedicated spot to do it instead of taking up another bench top? Great work with the welding either way, looks fantastic