... and of course, be ready for surprises! Bill's comment about the thickness being irregular is both true as well as tricky to deal with. Though the bulk of the transom on my boat has a 3/4" core, there are triangle doublers in the upper corners to support the backstay chainplates as well as a vertical one along the center for the rudder. There are also some thinner spots, notably where the fuel tank vent is. These areas taper to the new thickness at about 45 degrees, and where my ladder is mounted, hit both the vent and the triangular doubler areas. The original installation had fasteners in these 45 degree areas that were weak enough to allow the mounting weldments to move just enough to cause premature caulking failure. My solution was to use thickened epoxy to add G10 / FR4 tube with a slant cut on one end to bring the surface that mates with the washer up to level.

Good luck!