An oxy-acetylene cutting torch works by not just melting the steel, but also burning it. That is what happens when you press the cutting lever on the torch. Now, if you just set it to a neutral flame and let the molten puddle drip down, you would not have a problem with lead fumes, but if you use the normal cutting torch method, you're going to be vaporizing the metal in an oxygen rich environment.
A plasma cutter would also result in lead vapors.
The problem with a fast circular saw blade would be that it might heat up and you end up with your blade soldered to the chunk of lead that you are cutting. Plus, it's going to make a LOT of small lead chips. With a chain saw, I suspect that the chips will be larger.
Will your log splitter even physically handle the pieces? The cutting edge on the log splitter needs to be at least as long as the thickness of the lead that you will be cutting off of it.