Your SS got lost in the general resto forum or at least I missed it there among the hordes of rotten glassers. :lol:
Once stripped and clean you should to do any hull repairs needed, a close inspection to include the knee brace and leak test. After repairs and through cleaning of the areas Gluvit is to applied which is inside on the seams and keel joint.
If original then I'm not even going to ask about the transom condition and the work on it should come before foam and decking.
Good quality fir exterior grade plywood sealed with spar is the way I go for decking and transom. Decking 1/2" if you add extra supports or 5/8" if you don't. Either way you need to add better ply joint support than SC did with those flimsy joiner strips. Transom 3/4" laminated with Titebond III.
You're a long ways from adding deck covering but Nautolex vinyl is my suggestion and I used 3M 80 adhesive.