Greetings Shamrock,
You've discovered a gold mine. Not sure which 20 you have but if you haven't already done so, you might consider checking out Trayder's rebuild of a 20 cc in the Gallery. It'll give a real good idea of what your digging into. The other is to visit Strick's work in a thread near the top of the Repairs and Mods forum entitled "Here we go again..." Those are a world of info.
I'm a relative newbie to this site but what I've learned from these guys just by reading, doing searches, and asking questions is amazing. Great group of helpful and insightful people here.
Regarding your questions, the transom rebuild and addition of a bracket are common and well documented here. Well done transom fill w/140 zuke hanging off the bracket shouldn't be a problem. I think most will advise you to get maximum flotation bracket.
The grid and foam stringer thing has been done here and many would advise you not foam in the fuel tank. Original tanks were like 47 gallons or so but some folks have probably upped that dramatically. Perhaps even doubled that.
Stringers and keel tend to be pretty stout, and along with the hull shape, are the hallmark of the Seacraft. Transoms are known to become mulch over 30 years and thru-hull fittings, especially scuppers, will need to be addressed. But it's all well-documented here.
Now go save that SeaCraft.
