It is going to sound crazy but after soaking with WD-40/PB Blaster/Liquid Wrench try to tighten the heads just a little bit to break them loose. You can also spend about 2-3 minutes tapping each bolt with a hammer before turning them. I know tightening is the exact opposite of your objective but after soaking, then tapping, it frequently works.
If all that fails there is always the torch...
Good Luck!
Ken