I was able to get to mine
but it was not easy. I had a 7/16" wrench that I wedged up there and turned out the screws. Took a dozen trips in and out of the boat.
Some of the C25' have a brass plate imbedded into the fiberglass so there are no nuts.
On mine, I am going to have 3 nuts welded to the old gudgeon and use it as a backing plate for the new one. Haven't done it yet.
What some guys do is cut a hole in aft bulkhead of the cockpit and then they install a Beckson to finish the hole.
You can see it in this picture.
http://i9.photobucket.com/albums/a60/djnagle/IMG_2332.jpg