as you are about to find out they can be very hard to remove, first put penetrating oil on all studs and let set over nite, you can try to get it loose by starting engine with the nuts backed of a couple of turns from tight, if you cant start engine use plenty of penetrating oil, the best i have found is acetone and trans fluid mixed half and half, use a lead hammer, the type the machinest use, tap up on the head working all the way around, you can hit the head pretty hard, try not to use a pry type bar because you can damage the block and head and cause extensive machine work, if the engine is out of the car you can make a couple of eye hooks out of old spark plugs, you screw them in the spark plug holes and lift the engine a little with an engine puller while you tap on the head, have the head nuts on the studs two or three turns just in case the head comes free, hard work and paience? is the key words here, corrosion , warped heads and compositon head head gaskets make them very hard to remove, once you get it off some more fun begins it the head gaskets are composition, trying to clean the block around the studs is a nuckle buster, thats why i use copper head gaskets