Use a sawszall to cut the stringers back 4-8in or so, leave enough room to work. Replace your transom core, then you have 2 choices. You can either take the glass piecs you cut out and glass them back in. (my boats pieces were long gone so I didnt have this option) SO I took foam board and made a form that matched the existing stringer size and shape. Then I laid glass all over everything, 3in forward over the original stringer boxes, and tabbed into the transom. After everything kicked off, I drilled limber holes so any water that may get in outside the stringers will drain to the bilge.
Before I shaped them round on top, I had just dropped in and cutting to fit.
