Here is how I mounted mine.
Note that there is two things different from mine than yours.
One, mine has a headliner, and two, its a Ford........
But, if its double wall, it will work.
I took a piece of heavy steel, about 1/4 inch thick, 3/4 as long as the bracket, and about 2 inches wide.
Laid it on top of the bracket, and marked two holes in the center of the mounting slots.
Drilled and tapped the holes.
Drilled and tapped two other holes off from the bracket.
Drilled and tapped one more hole on the edge of the piece.
Laid the piece of metal on the ceiling of the cab, and marked all four holes.
Drilled them carefully through only the first layer, slightly bigger than the holes in the plate.
Took my Dremal tool, and cut a slot towards the windshield about an inch away from the holes, as wide as the piece of metal.
Put a screw in the one hole, and used it as a handle while I slid the piece in the slot.
I pushed it in until the whole piece was inbetween the two layers, and then pulled it back so it slid back past the slot slightly.
Then I lined up the two offset holes.
Put two countersunk screws in there to hold the plate in place, and keep the two holes marked by the bracket lined up with the holes in the ceiling.
Put the bracket up, and put two screws through the bracket and into the plate.
The radio covers up all the screws and the slot.
The plate is big enough so that the weight of the radio is spread out over a wide area.
