Do I understand correctly from your comments that you are in Alaska but don't haul your boat for the winter?
I changed all my gate valves in my 1978 27 footer this spring. It was a time consuming and fairly expensive venture but worth it in my mind. The gate valves looked brand new when I took them off but I still like the piece of mind I get from proper seacocks. They were in fresh water only.
Wait for Maine Sail to chip in here on this subject... it should only be a few minutes or so. He has an excellent site providing great instructions on how to do this. You shouldn't need to do any painting or fibreglass work but I used some West System epoxy to set my new bedding / backing plates.
Georgian Bay, Ontario
Catalina 34 - "Good Idea"