How to remove all lead to allow welding?

I am another vote for just filling the holes with fiberglass and leaving the rest alone.
Not necessarily because of the lead, but because it soooo much simpler and will basically last forever.
The Bondo Glass product works great for this kind of repair.
Used it to fill the factory antenna opening and emblem mounting holes on this fender, and years later can't even tell they were ever there.