Charles,
My tuning block for the Twings are directly aft of the aft most shroud on each side and just inside the toe rail on the deck. They are through bolted in the deck. The jam cleats are on the edge of the cabin top and are also through bolted.
At one point I had the jam cleat screwed into the wide forward part of the combing but found that the wood screws did not hold over time...through bolting is the way to go.
Good luck!
Robin