Any particular reason you are bending the edges of the side plates outwards instead of inwards? Last year we built similar side modules and found that by bending the tabs inward on the sides we could just flush mount the bumpers to the sides of the bot. Which was really easy and nice.
I would also recomend that you don't cut weight out of the pieces in front and back of the drive side modules. We seriously bent up the 1/8th inch metal right there on our bot, even without any weight taken out of it and a 1 inch piece of wood bolted in front of it to absorb the shock.
Another good way to attach superstructures to the frame is by making a big sheet metal bracket and bolting that to the top of your two drive sides and back side. Eg
