If you're going to all this trouble to get down to bare metal, I'd suggest welding in patches from the outside (underside), then sealing the inside original circular opening with seam sealer. The only lead filler I'm familiar with is the "old school" stuff used in Detroit long ago to smooth the outer body joints. This stuff was never designed to be structural, and I don't think it's available today. Frankly, even if it was, I wouldn't use it; lead fumes are quite toxic. Here is a lead-free product for use on vintage cars, but it, too is not structural.
http://www.eastwoodco.com/jump.jsp?item ... pe=PRODUCT