Wooo, New SeaCraft owner here :-) and boy do i have a project.

[Bushwacker]

I agree with FS on the suitability of plywood for the deck core! Check out this article for more info on potential problems with cored construction. If water ever gets to the plywood in just one spot, a large area of the deck will start to rot. You'll see what happens to wet plywood when you open up the transom! Moesly used balsa core for the deck and cap, and those parts on my 42 year old boat are still solid, so you could do worse than building it like it was originally built! Although balsa will also rot if it gets wet, the wet area won't spread as fast as it will in plywood. As Pasco's article mentions, balsa also has better strength properties than most foams and soaks up resin better, so it's easier to get a good bond, and it should be cheaper as well. Among all the foams, he seems to like CoreCell the best.