[Bigshrimpin]

From the outside . . . leave a nice fat lip. I wouldn't go as wide as Hammer, but enough of a lip so you glass the skin back you are glassing / fairing a flat surface. Dig out all the wood (a Drill helps see pic below) and then back fill the area with Arjay, seacast or some sort of pud/fiber mix. Once the skins are lined up with the new core . . . grind a nice wide 6" taper (3" each side) to the middle of the seam and use 4" 1708 tape to build it back up. Overlap tape across the seam (deepest part of the valley). The nice part about backfilling the bottom area with puddy is that you don't have to worry about penetrating the core if you mount a transom ducer