You dont have to have the torque box reinforcements welded in. Get some grade 8 hardware and bold those suckers in with an air tool, and they wont move at all. I guarantee you.
I was skeptical about not having them welded in as well. I beat on my car all summer and they not even as much as cracked the paint I painted them with after the install around the edges where the flat plate bolts to the floor.
Welding up the torque boxes isnt necessary either. Its extra insurance, but if your subframe connectors are steedas, like mine, they come with a torque box reinforcement plate you weld in the torque box area, then weld the subframes to it. Welding the thin sheet metal around the torque boxes could end up weakening it if not done properly.