It is a 40 year old boat. Keep that in mind. A well cared for and stored 40 years old boat should still be solid and IMHO worth keeping.
People seem to have the misconception that fiberglass boats are waterproof. The polyester resins that are used to make them are porous and if a boat is left out to collect water, in time it will get into the wood of the deck as well as stringers and bulkheads. I know as my 18' center console had it happen to it. If the boat was made with epoxy, this is not the case but that is the exception rather than the norm. What region is the boat in? Cold weather environments where moisture in the gelcoat and fiberglass can freeze exacerbates the problem.
The floor sounds solid. Is the gelcoat in good shape? A good looking gelcoat says a lot.
If you were to restore it, the stringers and bulkheads are not tall like in my Deep Vee. Material wise it is simpler and cheaper.
Post some pics.