I have just finished that same project!
I have just finished installing a chain plate for an inner stay on my O'day 37. It was a lot of work ,but it worked out better than I hoped for. I have have created a bulkhead using a 1" thick Iroko board ($20) that I spliced and lab joined with thickened epoxy such that it has a arch shape and has a long edge to attach to the hull. The bulkhead is slotted on the sides and epoxied to the hull. I have placed it right against the chain locker and bolted the chainplate to it, making it easier to epoxy in place. The toughest part was to remove that discusting close liner and clean the hull before tabbing. (I will use "goof off" next time: it made my life much easier once I found out about it!) This whole operation will also provide you with an opportunity to add stringers to the haul, insulation in-between and wood planks. Did I mention the bulkhead and the chainplate look great when you enter the boat?! It was a lot of work and would do it all again. I am not a rigger but I am a structural engineer, and from a force point of view, the bulkhead is enough to take the vertical portion of the load. The lateral component will be transfered from the chainplate to the deck (you need to make sure the hole for your chainplate is not too big. I saw a new caliber 40 that did not use bulkead to attach the chainplate but I do not like the idea.