Picking up on your earlier reply to a question from me - you said that you would bolt the mounting plates to the top beam rather than weld them, to avoid distorting the beam. Does that mean that you would not want to weld the beams directly to the upright supports, even though you are using epoxy to level the top surface afterwards? I had been thinking of tack-welding the whole frame, then carefully running full welds in progressively.
Maybe once I've started welding less critical bits of the support frame I shall get an idea of how much distortion to expect, but at the moment I'm trying to learn from the experience of others!