I think that will get you flat, but I wouldn't leave screw heads in place. I would grind them out and fill after the transom is rebuilt. Even more so if they aren't stainless. moisture finds its way into places you wouldn't think it can. Rusting metal swelling inside your glass isn't optimal. An ounce of prevention.
Humm...I thought of that too...I was wondering how I could lay up the first layers of glass straight before putting in the core. Do you think I could lay up 3 layers of glass, with the transom skin all wavy, then put core in and clamp it, and it should straighten everything out?
You should be able to put 3 layers in and let cure, then come back and sand a bit before gluing in the core. I wouldnt try to do it wet on wet,, too much going on. Those 3 layers wont lock the shape in. Once the core is glued in then your pretty much stuck with the shape/fairness...
Marcel4t, are you able to see the pictures in the links I posted above? If not, google "Photobucket patch" and install for the browser you're using.