That's a good looking bench Jay. The overhang will be handy for clamping things down. As has already been said, treat the bottom of the plate to prevent rust so you can flip it over a few years down the line. Tack a small plate to one of the legs for testing the welder on, use small tacks so that you can break it off and replace it every so often, I really do hate to see strikes all over the bench top! As for diagonals, forget them, a steel bench doesn't need bracing like a wooden table, the rigidity of the welded joints and the stiffness of the materials used in it's construction make them totally un-necessary, I've been a welder for a very long time and I can't ever remember seeing a braced bench.