They are fairly easy to change and you do not have to remove the heads. You do have to remove the rockers, locks, retainers, and springs. The easiest way is to remove the rockers, use air to pressurize the cylinder through the spark plug hole (holds the valves in place), compress the spring (careful not to push on the valve stem too much so it drops into the cylinder), remove the locks (magnet works great), remove the retainer, and lift the spring off. Pop of the seal and replace it. Here are a couple of pictures of my GT40 engine when I was changing the springs. The object in the second picture is a magnet to catch the locks.
