I too own a 74 10M.
You may have a delamination problem in the deck core due to water leaking around the deck fittings. Does the swelling feel "squishy" when you step on it ? The connections of the chainplates to hull are so massive I can''t believe they are the problem. If the core is water logged you might try drilling a series of little holes to let it dry out ( use a heat gun to acclerate) and then forcing epoxy into the core. Also, remove and rebed all deck fittings in the area looking for signs of wet wood. I''d try that before cutting open the deck and removing the plywood filler, although that is the preferred, albeit expensive method.