If he wanted half that price it would get my interest, but I've never tried to buy a big back window 58, so I don't know the prices or how rare they are. Around here that truck isn't worth that price.
I don't think the roof rust is a big issue, I would want to swap the roof cap anyways to remove the sunroof. However, you could always try and find a BBW parts cab to swap the back window from into the fleetside, if it's in better shape.
