lead filling

If you use the filler, it might help to put a piece of poly screen mesh (like for a screen door) on the back side to help the filler bond...if you can get to the back...after filling the hole use some filler on the back side and cover the mesh, like a sandwich. I did this 20 years ago and the filled holes are still holding

The only risk on leaving the holes open to the backside is rust under the filler. May not happen right away but, that is what I found on every open hole on this restore. Either weld or solder them up.