You people are confusing the sedan delivery with a panel/suburban truck.
The sedan delivery was build on a car frame, and yes there was a welded seam down the center of the roof. I don't beleive any manufacturer had the capability of stamping out a piece of metal that large. Don't forget there was no drip mold and so the panel had to go from beltline over the roof area and down the other side to the beltline.These vehicles were a low volume build so it was done the easiest way possible.
(I worked for the General in tooling for 37 years)
Stan Norrish