This is a project that looks simple but is anything but simple. I am not sure how a dinghy sling would work- for instance, what keeps the sling from moving under the hull?
I have a conventional Achilles dinghy and I mounted two eye bolts thru the transom near the floor and I glued two "D" rings to the inside of the bow. I mounted my davits and swung them inward toward each other to get the inward distance from the hoist points that you describe. A brace was attached to the davit arms so that they could not move.
I tied a bridle between the stern eye bolts and another one between the bow "D" rings and adjusted the length of the bridle lines so that when lifted the dinghy tipped to the side with a drain. I hoist the dinghy with the bow higher than the stern so that rain water would always drain out.
One challenge was to adjust the bridle so that the hoist hardware did not rub against the outboard or the bow tubes. Sounds easy, but it took me hours of testing configurations to get a working plan.
Finally I added four straps from each "corner" of the dinghy to the davits to prevent any movement and chafe. The davits and the dinghy are nearly a solid unit.
Take a look at the attached pix for the details I have described.






