Hello, Have fought this problem more times than I care to remember.
Some come off easy if the gaskets have been changed before. First, start soaking them with PB Blaster several times a day for about a week before you are planning to remove and make sure you drive the car. The PB and the heat / cooling processes will help any rust break free. If you have studs
and not bolts start buy using a well fitting 6 point socket and start gently loosening them. Then tighten ever so slightly, repeat this over and over until the nut becomes turnable. During this process keep soaking them with PB while you are working them loose.
If they don't com loose you can take a dremel with a cutter and cut a slit on each side of the nut being carful not to cut into the studs, then with a chisel or screwdriver you can break the nut off. If you have bolts
it's a different story, The problem is the PB doesn't want to flow up into the head where the bolt screws into.
Here's what I did, with a heavy duty hooked carpet or floor knife I dig out the gasket material around the bolt as much as possible, Then spray PB in them while using an air hose to try and blow the BP up into the threads. Then you can start trying to break the bolt free using the above method.
It is very important whether using a socket or a wrench that you do not apply hard pressure at a weird angle, this will snap the bolt or stud. Also, I use all studs on mine, that way if I have to replace the gaskets later and the nut is stuck I can just cut it off with a dremel.
Hope this helps! just my 2c
[This message has been edited by Ponnari (edited 05-19-2015).]