PS I am thinking to just drill from the deck side. In the fore cabin I am just going to cap the bolts and leave them exposed. In the heads I think there is a double skin so will have to make a hole in the inner skin with a hole saw to get the penny washer and nut in and then cap the holes somehow.
What I did when I did the mainsheet track in the cockpit was order every length of bolt from orbital fasteners which come in 5mm increments so ended up with the perfect lengths!
Rob
Parker 275 Number 7 Nimrod
Port Edgar Marina