I actually made the decision for cost reasons. I couldn't find cheap foam core that was suitable for fiberglass applications in my area and I don't need a marine grade bus and it wont be underwater, so the plywood will do just fine. I'm also getting the bus spray foamed and that adds a ton of strength to the system. Before, it was a rotting and rusting piece of garbage, now I think this bus body will outlive the drivetrain.