There are a number of options. Put the plugs in and turn the engine over. The compression might break it loose. Some people use hardwood wedges driven around the edges to break it loose. Another option is a tool either bought or made which sits on top of the studs and uses bolts going down into the plug holes to put upward tension on the head.
You should also post a message on sailnet's A4 list since a member of the list named David just recently went through this process and he could give you other advice.