any metal shops in your area?? perhaps for a nominal fee they would use a shear to cut it for you.
barring that, i would make a blade out of 1/4 inch steel and use a 20 ton or larger press to shear it myself.
easy peasy that way. might be you have a press, or a pally does. now just make a blade.
Funny you should mention the press. I was jacking up the front end of my old 4-door crew cab, and it occurred to me that at 7500 pounds, empty, I might possibly position the lead slab on some 2x4, then a cutting blade welded to a square plate, then a biggish bottle jack, then the truck bumper (3/4" welded steel plate box girder). The only difficulty would be getting and keeping everything lined up - so it's probably not a practical solution.
A large press with a knife tool might do the job, though.
In the interim, I also came across the following discussion on the same subject on a U.S. board, THR.
ht tp://www.thehighroad.org/archive/index.php/t-469958.html
As with my question, a variety of answers were suggested (except for the hot-wire cutter - I think you'd better patent that invention!)
A number of posters reported using a Skilsaw with a carbide tooth coarse wood blade, putting the slab on blocks and a tarp underneath to collect the shavings, which can be added in when melting. This method seems to have worked successfully for several people, as has the "axe" method.
I hadn't considered using a really large container and attempting to melt the slab - even partially - with the slab in direct contact with the container over the propane burner. I suppose the lack of a sturdy pot that size caused me to overlook it. A really large fry pan or something similar might work for this application, even if it only allowed removing part of the slab.
Henry Nierychlo mentioned that he has melted slabs directly into an iron pot using a tiger torch - perhaps I'll give that a shot before I break out the axe or the saw blades. Even if I could melt the corners off, it would help a lot in reshaping the slab to fit into some kind of vessel for melting.