I've never built a panel'd car, but I imagine if you use 3M 5200 you could probably be assured of both water tightness and some additional stiffness to the chassis....may be able to use a few less rivets. Just an idea.
Do not forget the proper surface preperation (fine scratches on surfaces to be bonded) and the right primer when necessary. I always use Wurth and always use the correct Wurth primer to suit. A Cliff indicates less riviots is OK and fitting panels with no riviots is ok ,just like fitting a window in every opening of the chassis. Geoff