The cabin top is definitely flat enough for long handrails. Wooden rails have some flex that allows for a slight curve. You may have to file down the ends so they will lay flat on the fiberglass, but not much. Since they are attached with a nut and bolt you can put more pressure than just a wood screw. I say get them as long as you want and they will work. However, I never really used mine as hand-rails and was just about to remove them when the boat sold.
