Wow - For us yanks that's a 5+ inch lift
on a 6 1/4" base. That's an interesting problem. I have never built boats but as an engineer
I can say that you are going to have one hard to stablize winch tower with 5 inches of leverage on the beds. Have you calculated how much moment you have at the base of the towers with a fully loaded sail?
Can you post a picture of the mounting area. It might help the construction guys get a better idea of what you are trying to do.
I might consider routing the sheets