Penetrating oil, like liquid wrench, put it on, let it sit, get it hot with a torch, or just running it, use six point sockets to try and remove(less likely to round off). Use a wire brush to clean it up a bit before you attempt all this. If worse comes to worse, you can always just cut them off. You should be using new bolts anyway, they should send new ones, but if they don't get some yourself. Go to a hardware store and buy some stainless steel nuts and bolts, or like for a marine application