I ordered replacement panels for basically a whole rear cab for a window truck that I planned to adapt to the panel truck body. The first step would be the roof panel. So I removed the outer skin for the roof.
The replacement roof panel had been damaged by the shipper with some substantial dents. I was able to hammer and dolly the damaged roof panel back into shape the best I could before attempting to install it. If I remember correctly, I also had to modify the middle roof stiffener that was still on the truck (runs from side to side in approximately midway on the door.) The panel truck roof-line was taller than what was on the truck so I removed the tall stiffener, modified the end trims on the stiffener and lowered it to better match the new roof contour. I also treated the rust on the roof substructure before final installation of the roof panel.
At this time I just attached the roof panel to the cab with sheet metal screws and used some wood board I had laying around to support the back portion where the window panel will eventually be installed. I will also note that I do not plan to install drip rails so that “should” simplify the effort.
1949 Chevy Panel Truck A Project Journal I'm old but I make up for it by being immature.