Welding table help

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Picked up a nice welding table on craigs list. The legs and sub frame are made of rectangular tubing(rack/shelving stuff) and the top is 1/2 inch plate. I cut the top plate off the sub frame and legs(I'll use those later on another table) I was just after the 1/2 plate, got it for a good price but it's a little concave(bowed) in the middle, about 3/8 of an inch. The plate measures 72 x 48 inches. My plan is to make a new sub frame out of 2 x 4  (0.120 wall) rectangular tubing. The legs will be 2 x 2 (0.120 wall) rectangular tubing. See CAD pics for dimensions and layout. This layout leaves a 5 inch overhang around entire table, plenty of room for clamping. I'm considering welding nuts to the underside of the table and tightening down the table to the sub frame using all thread and some tabs welded to sub frame, hopefully it will get the bow out of the plate and secure the plate to the sub frame/legs. My main concern is that the sub frame and legs stay straight and square and the plate flattens out. I thought about using heat on the plate to help flatten it out as I'm tightening the nuts. Maybe adding some 45 degree gussets from sub frame to legs, and thicker wall tubing might help too. Any comments or ideas would be great before I buy the sub frame metal.