Clamp them in a vise but you'll need an oxy-acetylene torch. In my experience a propane torch (like a Bernz-o-matic) doesn't have enough poop to do the job. Heat to glowing red-to-orange and be ready to start wrenching as soon as you take the torch away. As a knucklehead, I've done the heating and realized that I didn't have my wrench ready to do the deed.

I'm wondering if someone has been there before you and installed steel nuts. The copper (or copper plated) nuts shouldn't seize like that.

Also - I prefer the copper-based anti-seize if you can find it. The silver Permatex Anti-Seize is OK but not as effective as the copper-based stuff. I get mine at a bolt house, like Fastenal.

Dan