Good plywood is probably the best for the bulkheads, marine ply like occume or meranti the best. I would use a bit of foam as a spacer between the hull and the bulkhead edge and fillet and glass, using biaxial roving and epoxy. See diagram below.
I am not really sure what you mean about the chainplates as there will have to be holes for the bolts through the deck anyway.
There is core in the keel area?
Living aboard in Victoria Harbour