If it is a long wheel base you can, as this is how the fieldman is created. As to where I'm afraid the usueal. Main dealer, breaker or e-bay. Probablr eisier to find a whole rear roof with van pannels in.
As for the SWB. I'm guessing any flate sheet steel could be cut and welded / bonded in, as that is all there is in there in the van.

Any veiws expresed in this thread by me are purely from my own experience, and (sometimes) falible memory. Hope my comments help, but please don't take them as gospel.