Yes, mine is a dodge, but I believe if you see roof gutter rust mostly on them, it is because there is more of them floating around.
Here is the convoluted upper rail and gutter cross section of a Dodge.

I imagine the fiberglass roof is made one piece in a mold, I am not envisioning any benefit of having 2 pieces and making a seam, not in something the size of a van roof.
If you buy a van and cut the roof off, I'd recommend treating any rust and repainting it. When cutting out the steel, I'd recommend painting the edges of the bare steel. Use stainless steel fasteners, but predrill the holes vacuum out the shavings run the screws through the steel to cut the threads, back them out and paint the holes.
Leave enough room so that the fiberglass roof is not resting on the roof gutter bottom, partially blocking some of its width.
My roof, with nothing attached to it weighs about 220Lbs.
If cutting the steel roof with a cut off wheel, protect any of the vans paint from sparks which will sear into the paint and cause little rust boreholes in the paint.