I used to do leading, and still have quite a few sticks of lead. I used 30% tin 70% lead. I think plastic filler started in the late 50's or so. The last factory lead that I know of is where the windshield post connect to the cowl on AD pickups was leaded. Some people call it soldering, but it is a little different. Soldering requires enough heat for the solder to flow. Leading requires a little less heat. I have a tip that I put on a torch that uses acetelyne but no oxygen. It draws air to burn and burns with a green flame. The lead is heated to where it is kind of like cold margarine and is spread with maple spoons while it is in kind of a crunchy state. Having your metal very clean is important. I used to do a lot of leading when dechroming a vehicle was the thing. I would lead the holes shut.