I too would add something back there, both between the frame rails and another plate outside the frame rails to allow for another set of rear bolts. When you bolt the rear wall down after the plywood is in, we took advice from a few forums members and ran a piece of aluminum stock (1/8 x 1.25) down inside the C channel to provide a nonstop washer with a lot more structural integrity than the sheet metal channel.
Scott & Megan
VAC LIBRARIAN WBCCI 8671
1963 Safari from the 1963-64 Around the World Caravan