Late to this post as I just joined this forum, but seeing it as a helpful topic for others, sharing what I know. I can only speak for the build of the 1984 290 XF that I own and personally have cut into:
1. 3/4” Balsa core was used for the forward bow decking. I replaced mine with 3/4” 2lb. polyisocyanurate foam.
2. The stringers are made of wood. Replaced one under fuel tank.
3. The transom is glass with a very thin foam core (but no wood).
