If there is steel sheet outside the lead, cut a slot for the chainsaw or power saw with a 1mm blade on a mini grinder. That is how I would approach it.
I have used a power saw on lead with a tungsten blade. Got to have a strong hand tho'. The chainsaw is more forgiving.
Forgive me if this is old news, but remember lead dust is toxic and once it gets into the lungs it doesn't like getting out again. Wear a good quality mask while you are cutting the lead, and try to stay upwind of it.
Of course you'll also tent the area, so you don't contaminate the ground. If you're in a marina or club--a lot of them are dead serious about lead contamination.