With the engine going through hot and cold cycles through it's life, it is not that the screws are too tight, but that there is corosion due to electrolosis. Your best bet would be to use some WD40. If there is a gap between the head and the cylinder, spray it in there and give the head a tap with a hammer. Do this several times, it may take a few days to get the WD40 to work into the threads.

Also when you put in new screws, coat the threads with antiseize.