[DeTomaso] Reduce Weight or Stiffen Chassis?

[Art Stephens]

     I am eliminating the fake vents behind the rear quarter windows.  The area will be covered with 18 gauge sheet metal that will be welded in.  My question is how to fill the holes that the studs pass thru holding on the vents?  I want to fill the holes to keep water out of the soon to be enclosed area.  (I will have a drain hole at the bottom of the area).  My car does not have the side gas filler.  So I can either fill the four holes with metal discs,  welded in,  or I can open up the gas filler hole and then cover all five holes with plastic plugs glued in.  Filling the holes with metal should theoretically make the car stiffer,  using the plastic plugs will make it marginally lighter.  So what is the best solution?  

Art