For help on the hull/deck joint you might try
http://plasticclassicforum.com/
as these guys have discussed that particular issue at great length.
Why would a rigid vang break a boom? I see no inherent danger in them, but like anything if used improperly...
I always think of boomkickers as a rigid vang on the cheap (and light I suppose.) There's nothing in them that is easier/safer for gear. In fact for cruising I'd be a little worried about the boomkickers limited travel; when you ease them too far out the boomkickers fiberglass rods fall out of the bracket on the boom, and have to be reinserted before it'll function again.
Under what conditions were these booms broken?