Paul, per your bio here, you own a Watkins 27.
As pointed out by others, moving the attachment point for the mainsheet to the cabin top increases the loads on the boat (and boom) by a square-root sort of force increase factor, i.e. a Lot.
The thing is, as perfect for your needs as your boat is, that particular company never (...trying to find some calming words here...) put an ounce of material or a minute of engineering into their boats that was not required to market them successfully to the first owner.
Take the comments from experienced owners here to heart, and don't ask more from the structure than it was built to give.
Just enjoy your boat and sail it a lot, and try to sail in protected waters.
All boats, including heavily-built blue water designs DO have their own limiting "performance envelope" and all will have their failure points...
Hard Truth: many craft at the lower-price end of the production boat scale just have a much more modest limit of wind 'n' wave.)