I had a 73 Mk1 Gulfstar for about 3 years. The hull is solid glass, and almost bullet proof. The decks are balsa cored. Expect to find spongy spots anywhere hardware was installed, either screwed in or through bolted. The fittings will leak, and the balsa will turn to mush. If caught early enough, the repair is simple. We removed all our stanchions from the main deck. They were wobbly and you could see the deck compress. To remove the bad wood, I cut the head off a nail and bent an inch of the pointed end 90 degrees. Installed in a battery drill, you can undercut the mush around the fastener hole. Clean out and dry hole with a shop vac. Seal the hole from the bottom with duct tape, and fill with a slightly thickened epoxy mixture. After it has set up, remove tape, and re-drill holes for hardware. Re-install stanchions with a liberal application of caulking. A few of my spots were more extensive, so using a Rockwell oscillating cutter, I removed the deck back to solid wood. Clean up the hole and fit a plug of exterior plywood, 1/2 inch in this case. Locate the bolt holes on the plug and cut them oversize, about 1 inch. Seal the underside of the deck with duck tape, and install plug with thickened epoxy, filling the oversize holes. Clean up the deck piece, epoxy in place over plug, re-drill holes and install hardware with sealant of choice.
The fly bridges are notorious for being spongy as well. You can fix them in sections, or if really ambitious, remove the bridge and completely replace the deck. Note - that will require supporting the deck from the salon below. You can follow people on U-tube having done this to boats of other brands.
Hope this helps.
"Keep putting off till tomorrow, and you'll end up with a lot of empty yesterdays" Prof. Harold Hill