Seat Bolt should be very secure especially in a hot rod. I would go with flat bar stock like you're talking about, attach that to both ends of the body rail frame, drill and tap for truss head machine screws instead of rivets. Drill the holes for your seats (and seat belts) then use cage nuts, you can weld them on or drill and bolt them. All this is a little extra money and work but way better in the long run for ease and safety.
