a long time since you posted but here's my 1/2penny worth! I have a tartan 30 & have done various jobs including replacing the stbd bulkhead. The hull
is solid glass, thickness decreases as you move up from the keel. Deck
and coachhouse are either balsa cored or ply cored depending on location, it seems that flat locations (like either side of the companionway
hatch) they used ply and on curved locations balsa. I will be interested to hear your progress and exchange notes.