Like mo dingo said, at the ends. Personally, I'm going to weld mine at every possible point, with one long, continuous bead. I don't ever plan on taking them off once they're on, and if I really wanted to, I'll just go through several grinding wheels to do it. The more welds, the more they'll stiffen the unibody in theory, although I haven't seen anything that says anything either way. Just weld them, instead of bolting them in, for a much stiffer chassis.