Installing body plugs in 68 b body have 10 smaller rubber plugs that fit the same size as the plastic ones where should the rubber ones go?

As a general rule,the rubber plugs go where they are exposed to rocks, and debris from the bottom side,so that they stand up to rocks etc hitting them.The plastic ones go where the bottom sides are NOT exposed to abrasion,such as the holes into the frame rails and floor areas that are boxed.