I'm quite sure that I mentioned using a propane weed burner for melting down large chunks of lead, of this size and much larger. Stay clear of using oxyacetelene because it burns far too hot, vaporising a lot of lead. The weed burner flame is cool in comparison, but extremely high volume in BTU's and it will melt large chunks rapidly. A neighbor fixed a large melting pot, I believe around 18-20 inches in diameter and two feet deep. He set this on used clay brincks, dry stacked in a circle to form a wind break and contain the heat. He blasted a 400,000 btu weed burner into a hole left in the bottom for about 45 minutes to melt down one ton of lead. All venting was thrugh the natural cracks left by dry stacking the brincks.
I've thought about using a chinsaw many times but will caution that if you do, fasten the ingot down tightly, and be sure the rakers aren't cut down much, to minimize grab. It would be wise also to sharpen with a maximum angle of about 10 degrees as is done when chainsaw milling lumber, as the angled cutters of normal chain will tend to pull sideways cutting large gouges in the side of the cut. The gouges don't matter, but the extra cutting is a waste of power, more grabbing and a waste of lead.
Non ferrus carbide tipped metal cutting blades are available for both skill saws and table saws, and they cut smoothly in aluminum, lead etc. The teeth on these are sharpened alternately, one square the next V shaped with a flat end and this tooth extends out a few thousandts past the square ground teeth. The V tooth releaves the center of the kerf, while the square tooth removes two slivers , one each side of the kerf, and holds guage on the kerf. Even these work best with some kind of lubricant. Diesel fuel works well but stinks, WD40 is good or a soluable cutting oil designed for water mix, mixed per recommendations, works very well to prevent blades gumming up.
Avoid any saw that is very fine, as it will turn into a lead plated glob before you get a half inch into the lead.
I knew of one man who sawed a huge lead weight, I think he said close to 1000 pounds, up with a bow saw, the kind made for cutting firewood etc. He had a lot more energy and patience than I do.
Trash all the above methods except the weed burner. They cost only $50 to $75 and are unbeatable.