Ericson- Mast Compression Problem
To start with a 1/2" of movement is very large for a simple problem of overtightened stays and shrouds. I would normally think of 1/8" of deflection as being a pretty large amount of movement without other problems.
I actually raced on an early 1970''s vintage Ericson 35 for a couple years in the late 1970''s and so have some familiarity with these boats. Ericsons have achieved something of a cult following these days but dispite being a bit heavy by modern standards, in their day these were seen as incredibly lightly built boats. In the 1970''s there was not the kind of engineering of boat structures that we can now do and so, while boats of this era were pretty heavy, they were not especially stiff or strong by modern standards. Ericsons of that era were known for resin curing problems that also affected their strength and stiffness.
Beyond that, 35 knots of wind, and the waves that typically accompany that much wind, is a lot of wind for any boat and can do some damage even to a reasonably well built and maintained coastal cruiser.
Without seeing your boat it is hard to say what has specifically happened but some likely candidates would be as follows:
-Rot in the ends of the mast compression post or separation of the mast compression post from the bulkhead. I would look in the bilge at the bottom of the mast compression post or at the deck level.
-Movement of the bulkheads. I can''t recall but I think the chain plates were attached to bulkheads. Loads get placed by just tightening the backstay which can "banana" the boat and pull the tabbing from the bulkheads and spring hull to deck joints. I would check the bulkhead for detached tabbing from the hull or bulkhead and for rot at the hull and deck.
It is possible to tighten rigging on an old boat suficiently to twist the boat completely out of shape. This usually involves a lot of tension all around and a boat with other inherent problems. A careful search of interior furnishings would show hair line cracking at the hull tabbing.
These are all pretty major issues but they all are repairable. I need to leave for the office. Good luck