I'm only guessing here, but I imagine they don't allow subframe connectors? I am not building to compete in that class, I do have subframe connectors, but as usual I got to thinking. I'm still doing sheet-metal work, and am replacing my rockers. I know it's a box section, but I didn't like how flimsy the inner rocker felt once I took the outer shell off. It's a 68, and there is no bracing in the door panels and I was thinking if anybody ever drilled me from the side, I don't have much protection. I am welding some one and three-quarter inch square unit strut to the inside of the rocker. I might even run a second piece. After I welded it on, I closed the open side with a piece of three-quarter inch EMT. Then as I was working I was thinking that if somebody wanted some chassis stiffening without subframe connectors this may be a way to go, of course it involves a big job, but what the heck. Yeah it's ugly, I don't need comments about that.