If it were me and my boat I'd go with 5/16". After finding the best area to work after looking inside the v I would then drill the holes, outside the v to inside using the u-bolt as a guide. After seeing where the holes come through on the inside I'd make that area flat to take the long thin backing plate into the v by using a mish mash of fiber from mat by pulling it apart and mix it with epoxy
to make a flat area to take the backing plate. You'll be covering the holes you've just drilled but the goal is to make a flat area for the backing plate. After the mish mash hardens I would then drill through again, take a measurement of how long the bolt needs to be and use or get the 5/16" u-bolt that meets that requirement. Once you've gotten to that point, use either 4200 which will allow you to remove the bolt later easier or use 5200 which is a more permanent but less flexible adhesive sealant
and push the u-bolt through after gooping it up thoroughly, go inside and install the backing plate and twist on the bolts that have lots of goop on them and tighten down to just enough to let the goop ooze out all over. Clean up the mess. Come back the next day and torque down the nuts on the u-bolt enough to feel like about 45 lb ft and it's done.