JBarca is right on. I'll add one more suggestion. If you have the thinner tube frame you may want to do what I did.
rather than trying to bolt into the frame, i made a 1/4" plate with holes drilled and nuts welded to the plate that would line up exactly with bolts through the bracket. plate was about 8 inches long and almost the height of the tube. Then drilled clearance holes through the tube frame for the bolts. Now the tricky part. how to align the plate with the bolts for the bracket to clamp everything in place? well I also welded a nut perpendicular to the plate and attached a threaded rod to it. then I used the threaded rod to locate the plate to line up with the holes in the tube. then using grade 8 bolts, attached the bolts through the brackets to the plate. then tighted it down tight. now the bracket is attached to the tube with a large thick plate to distribute forces and clamped with real bolts and nuts that can't pull out.
this in some ways mimicks the channel type frame that JBarca has.