Originally the fwd dome light just screwed into the masonite headliner just forward of that roof support. The hole on the front side of that rib is where the wiring comes through, the wires go through that rib to the DS then over the door and down the inside of the A-pillar to the dash. That sheetmetal U bracket on the roof was to support the center of the headliner with a screw and beauty washer. I have that but mine is a little different, it's attached to the center of the next rib fwd. Yours doesn't have that rib, older van?. I did the same thing as you and got what you have which is a cargo area headlamp bracket and attached to the rib just as the cargo lamp does. That is the same place the light attached to the headliner. Hope that helps. I can see why your confused b/c of the camper top you don't have a cargo light to compare. I can post a pic later at lunch.
What I did in lieu of the headliner:
The stock cargo light:
I have the original headliner, but I want to remake it. The light was just hanging from the wires when I got the van, so I got one of the cargo light brackets to fix it till I get the interior done. I'll probably keep the bracket even with the headliner though b/c it seems more skookum to have the headliner sandwiched between the light and the bracket. I'm also going to get rid of that U and fix it to the rib there.