Get a good penetrating oil like Marvel, or even better KANO KROIL if you can find it. Soak them with penetrating oil on the head of the bolt, and at the mating flange, every day for a week. Then drive it after soaking. The expansion and contraction will work the rust loose.
When you go to remove them if one is stuck, work it back and forth just a tiny tiny bit. If it wont come out soak it some more, put the others back in, and drive it more.
I did this on mine and it worked great. I also used antisieze on the threads at reassembly.