Speaking about just the bullseye fairlead, it appears that you should drill (tap) where you want to place it, using a small bit to drill to see if wood actually comes out, which means that there is a wood support (ridge/spine) under the deck, since there is no port opening to look or feel from underneath. Even if there isn't any wood, then you should still be okay if you use 5200/4200 (silicone based sealant) to bed the screws. You will have to use screws without nuts since you can't reach in there to fasten from underneath the deck. Try to match the same threads as the picture, there are no nuts on those screws either, so you should be fine.