The huge nut that locks the flush valve assembly in place on the underside of the tank is melded with the threads. We've done wd-40, heat, CLR, and vinegar. We're looking for any other tips/tricks to free the nut without destroying the tank.
Cut it off with a hack saw? Many plumbing parts are just brass and a hack saw will go through one quickly. It might scratch the bottom of the tank a little, but it should not shatter it. Another option would be to cut through it with a chisel using gentle taps from the side (force parallel to the bottom of the tank). Once the nut is in pieces you may be able to pry them off one by one. There is no picture, so this may be wildly off the mark, but sometimes the part on the other side of the tank is easier to cut through than the part sticking out of the bottom.