I'm surprised to hear of screws being use for chainplate on a boat of that
size..

IMHO you should go ahead with through bolts..

You can first drill from the outside with a long 1/4" pilot bit, coming all
the way through the ceiling. This will give you an accurate location. Then
take a large hole saw that you can handle and cut an access hole (don't go
too large, where the holes might over lap) in the ceiling material, all the
way to the hull planking (stop there).. Install a backing block as large as
you can fit into the access hole. Followed by the new chainplate bolts and a
large fender washer and standard washer and nut.. (I don't use nyloc or lock
washers, I just put a dab of 5200 on the theads.)

You can do a very neat job of plugging the access holes by turning some
nice hardwood plugs with a ridge to keep them from going in too far.. Secure
your plug in place with a dab of adhesive (you will want to check your bolts
periodically without destroying the plug).

Now you have a properly secured chain plate, with the proper fasteners that
can be inspected periodically.


--
My opinion and experience. FWIW

Steve
s/v Good Intentions