This poses some interesting point's as the tub on a Seamark is different.
Here is the file from D&D that I used. Don has now put it on his website, so I will show it. I kept this file offline in the past.
http://www.dadmarine.com/doc.htm
He told me after I mounted my bracket that the bottom should be 3in up from the keel.
But that doesn't jive with the rest of the install imfo.
On my boat [different from your SeaCraft's] My motor's are raised to use the bottom motor mount hole [or will be] they are in the next to the bottom hole right now and the anti cavitation plate is still below the water surface by aprox 3/4in when on plane.
The tub size is lacking for 2- DF-140's and I'm thinking of building a bracket now.
http://www.classicmako.com/projects/xshark/bw19.htm