Soak the bolts with PB BLASTER for a day or so. Then try to break them loose with a big wrench, with a long piece of pipe over the handle to use as a cheater bar. You might have better luck in banging on the bolts a few times with a hammer, after they have soaked in the Pb Blaster, and before you try wrenching on them
If that doesn't work, heat the bolts with a torch, then use the wrench and cheater pipe to try to turn them.