I'm not sure that this is a good idea. If you do proceed, at least cure the salt very gradually with eye protection & a fire extinguisher at hand. While salt has a fairly high melting temperature around 800C, your salt slab isn't going to be pure. There are surely some impurities, excipients or different forms of salt that could seriously depress the melting temperature. Molten salt is nasty stuff. You may get flying pieces too if there are any trapped pockets of water or air & it's a relatively soft "stone" to begin with.
Bottom line, I wouldn't risk it in my kitchen & recommend that you use a cast iron skillet or pizza stone instead. I'd hate to hear that you tried it with bad results.