Hi Tom,
I am also in the middle of a transom job on a '74 23. I removed the original 2 sheets of 3/4" ply and plan on putting in 2 new sheets of 3/4 marine. Why the extra sheet of core? I think you'll find the experts here tell you the strength is in the glass not the core. But I'm no expert
Why not beef up the inner skin/lamination sched? I know my orig inner skin was only a layer of mat covered by a layer of heavy roving. I plan on putting more glass back in. What are you planning for power/transom setup?