How do fleetside rear bumper brackets fit?

[BigSix]

The previous owner of my truck had welded in a number of crossmembers and things in the back to fit his homemade "trailer hitch" (read: scariest contraption on earth). After much grinding and cutting I have much of it off and getting ready to install my LMC fleetside rear bumper with their brackets. I left in one crossmember that goes between the frame rails in the rear. My bumper brackets just came in and I am thinking that the inside brackets bolt on the inside of the frame and the outside brackets bolt on the outside of the frame? So basically the bumper brackets sandwich the frame? Meaning I have to grind out that crossmember that goes between the frame rails...?