99 S80 Subframe Bolts

I got the parts to do the subframe o-ring job. After lifting the vehicle, blocking it in place securely, then bracing the rear of the engine, I was not able to remove the large rear subframe bolts as they were in too tight. I also notice that they seem to be 18mm. A rather uncommon size. Any Ideas? Do I need to remove pressure from some specific point on the engine/transmission? Do I need to apply a ridiculous amount of pressure to start the bolt turning, (I was concerned with breaking it as it was so tight)? I also noticed that there seemed to be a o-ring similar to the one I am trying to replace, just above the first rubber ring, (which looks fractured). Are there two, one on top, one on bottom. I have VADIS and AllData, There are close to useless.