I would purchase a "Berts" track system. Mount it to the top of your Gunnels. You can then purchase the different Berts inserts, mount your rod holders to them and slide them wherever you chose to have them. Gone are the days when you hard mount rod holders only to find out you don't like the location.
Berts is not the only company that makes tracks but is the company I am most familiar with. They make two different widths and several different lengths to get exactly what you are looking for. I've used them on my Salmon boats and two of my other boats. My current boat has a track system built in to the gunnals so I didn't need berts.
You can purchase them from BP, Cabelas, Gander Mountain, and several other companies.
I have OCD "Obsessive Crappie Disorder"