The Bolts bolting the rack to the shell are 3/8" carriage bolts. I filed the holes in the rack feet square for them.
On the inside I used flat fender washers and bent them to match the shape of the inside of the shell. There are 12 total bolts holding the rack to the shell. I used the rack to lift the shell off the bed of the truck to add new seals to the top of the bed, dust and wet control. IN YOUR CASE - with the thin sheet metal roof I would use a full form fitted plate of steel so it does not rip the roof. Our fiberglass shell is pretty thick almost 3/8" to 1/2" where I mounted the rack.
I put bolts, washers and nuts in the original extra holes in the shell I did not use. So I put hooks on them also and added more clothes lines for drying things.
The poles are extra legs for the Alu-Cab 270 awning.