The guy that runs Devious Customs did this mod to his square bodystyly crew cab dually...dont know what the window came out of..but I remember reading about the mod somewhere..try there website or there myspace page.
Its a solid flat glass so all you have to do is build the frame and get the glass cut. Hardest part would be the seal. The Devious dually is the only one Ive seen.
I remember seeing the one devious did(what ever happened to that truck?). I was just looking at a pic the other day and noticed how much higher the glass is than the top of the bed and wondered if they where all that way. After thinking about it I think Campy might be right about them already having a big window(hard to believe he'd be right about something).