I have also seen them done by using 1" shank bolts in th the bottom of the upright tubes and the bumper ends just weld a nut in to the tube or thread the inside of about 3 inches of a heavier wall tube for the 1" bolt then turn a sholder on the out side that will slide in to the other tube. like making a rod end insert. Then make a counter sunk location on your sliders or where ever you attach it to so that the bolt head is not exposed.