I don't think I would worry too hard about those, get the wagon-van together first and see if you feel you need them. I know my 1980 van shell, with those in it, still oilcans and flexes... They can't be doing a whole lot. Maybe someone who does fiberglass can make a fiberglass set than can be bonded to the side and actually increase the side stiffness!
Chris
We currently make the van upper side panels in fibreglass which are stronger, less flex than the metal ones on the van (even with the stiffner in place).
We are also thinking about making the stiffners from fibreglass to go with the above panels, so anyone repairing a van which as been converted to windows or anyone wishing to make an estate into a van, can have both side panels and stiffner for strength/originality.
Jordie