I used the stainless bracket from Rex (http://shopping.rexmar.com/Merchant2...=swimstepbrkts) , along with their urethane full-width swim step rather than teak (less maintenance). It works bitchen, right at the water line, couldn't be happier. The bar for the cav plates runs between the diagonal support tube and the transom bracket.
Also talk to Dave Sammons. He has done brackets like you're talking about, and his work on Brent Spurlin's 24 Spectra is nothing short of awesome.