Not exactly your situation, but relevant. I installed flush mount Scotty rod holders on my Outback in front of the built in rod holders. In searching through lots of install information the one thing that seemed to come up a lot and make sense was to use some type of backing plate inside of the hull. I used and old cutting board. Thus, the bolts went through the hull, through the backing plate ( piece of cutting board), through washers and then the nut went on. The backing plate helps to better distribute the load and stiffen the hull in that area. I have not had any issues with my install.
The one thing you want to be careful with is to make sure that the lines inside the hull (steering, rudder up/down) are not impacted by the install. Feel where you want to install while you raise and lower the rudder and steer it to make sure your install will not impede their operation.
Best of luck!
Fish tremble when they hear my name
A ship in harbor is safe -- but that is not what ships are built for.
--John A. Shedd, Salt from My Attic, 1928