That's always tough. Wire brush away what you can. Maybe use a dremel or wirebrush attachment in your drill. Use eye protection when doing this overhead. You can dissolve the calcified deposits using vinegar and spray some penetrating oil (Liquid Wrench or PB blaster) to help with the seized bolts. Sometimes a little jarring will break it free too. Use a nail punch to direct the impact to the nut w/o damaging threads or bending the bolt. Depending on what you're doing, perhaps cut off the bolt/supply line below the nut and use a deep socket on your ratchen with a cheater bar. Good luck.