Great looking truck. The question comes down to seal and support, like a very long gasket or to support and float. The stake supports on your truck look strong and deep. The posts will be supported very well. The slats, just tie the posts together structure wise. You get to choose. No wrong way with the structure you have. If the post supports were short, then the rail and the slat would work to add support. You have nice deep supports so do what looks best to you.
