Seat belts have to be worn where fitted.I would think your van was built before they were a legal requirement so if its designed as 2 berth then it would be illigal but if built for more than 2 then you could carry people in the back but only to the number the van was design for.Possibly as a commercial based vehical it may say on your log book its seating capacity or I'd suggest a call to your local DOT