Taking some core samples all the way down to the bottom would be what I would recommend. I have yet to see one of these older hulls that don't have delamination in the forward section. Not sure if it was either the quality of the materials, proper prep of the hull before the stringers were tabbed, lack of the number of layers of tabbing or all of the above, but they seem to have issues more regularly than not. Even if you just took out a few sections to check forward and aft. I wouldn't personally put a deck down when you have an opportunity to check it. You could always fill the areas that you remove if all is well. 

Starting to come together now 

 

impossible to tell without seeing, but given the age, is it more than likely the deck, stringers and transom is rotted?