I reused the upper pair of original holes, filled the lower pair of original holes with thickened epoxy, and drilled two new  lower holes at the bottom of the mount. This gave me the greatest spread the mount bracket allowed between the upper  and lower pairs of holes. I used 2" by 1/4" aluminum strip from Lowe's as backing plates inside the transom, one strip for  each side. I also used SS fender washers behind the aluminum plate. I worked through the inspection port on the inside  transom wall. I removed the white plastic inspection port entirely, in order to have the largest possible hole to work  through. I covered the raw edges of the hole with duct tape so it wouldn't cut my arm when I reached through to work with  the plates, nuts, and washers. I bedded the bolts thoroughly in Boat Life white caulking compound. Don't use adhesives  like silicone sealer or 3M 4200 or 3M 5200. They could make future removal difficult or impossible.  This shows the position of the upper right swim ladder mount relative to the HIN.  I drilled the hole next to the HIN first, and put a bolt through it. Then I leveled the tops of the two top brackets, and  drilled the outer left hole. I put a bolt in it. Then, I drilled the rest of the holes using the brackets as drill guides. Measure  and level a bunch of times, drill each hole only once!  72