Through the centers of the chocks to the edge of the deck is 54 inches. The distance from the center of the cleat through the chock to the edge of the deck is 6 inches. The cleats are slightly forward of the chocks. For what it is worth it is 45 inches between the centers of the cleats.
The PO had one made for Windrunner. I sold it to Far Reach a Cape Dory 36. I do not know if he is using it. It might be worth contacting him and making an offer. There is a 'Far Reach' website.
Beef up the cleats. We put oversized backing plates under ours and had a stainless plate put under the cleat. The forces generated by the jsd are huge. Others have bypassed the stern cleats using chain plates placed horizontally just under hull deck joint to the distribute the load. The farther apart the better as the quicker the boat will align perpendicular to the next wave.