Well, first of all, what kind of car is it? Some cars have boxed frames stock.
Chevelles are what I know, so that's what I'll talk about
Chevelles have the side rails made out of 1/8" thick steel bent in a big "C" shape. The open end allows a lot of flexibility, which you don't want. By welding a plate onto the open side to close it, it makes it much more rigid and better for cornering.
I just traced a pattern onto cardboard, and plasma-cut out a some new 1/8" steel pieces to fit. Any welding shop could cut the pieces for you, if you don't have the equipment.
I'll look for some of my pictures I took of what I did.