That Pyro kit is extremely old; it was, in fact, one of the very first plastic sailing ship kits, originating back in the early fifties. I think it was copied virtually intact from a wood kit by the now-long-defunct Marine Models, Inc. (Pyro was known in those days among wood ship model companies as "Pirate Plastics.") But I really like it. It's simple, but the basic shapes and proportions are right. It's high on my list (a short one) of kits I recommend to newcomers in the hobby.
Ronnberg is one of the foremost authorities on American fishing schooners. I think the book you've got is in fact the instruction manual for a wood kit from either Model Shipways or Bluejacket. He's done several. The one that would be most relevant for anybody working on a Gertrude L. Thebaud is the one from the Model Shipways Benjamin W. Latham or Elsie. I have a horrible feeling that the one you have may be the book that comes with the Bluejacket Smuggler. That one's about fifty years too early. There's quite a bit of difference.
The other classic source on the subject is The American Fishing Schooners, by the dean of the history of American naval architecture, Howard I. Chapelle. It's a big, fat, hardbound book loaded with plans, detail sketches, and photographs. New copies are kind of expensive, but I suspect you could find a used copy at a very reasonable price on the web. Chapelle spent years prowling the docks of New England seaports making sketches and taking pictures. I suspect there's more than enough information in that book to build the sort of model you have in mind.
Take everything you see on modern replica vessels with a huge grain of salt. I don't know which particular ships you had the opportunity to visit; they vary tremendously in accuracy. The Bluenose II, for instance, looks just about like the original Bluenose from even a short distance, but fishing schooner buffs have no trouble finding anachronisms in her rigging and deck furniture. (Part of that problem, it should be said, originates in modern Coast Guard safety regulations. The Thebaud couldn't pass a safety inspection today.) And when it comes to such things as the appearance and color of rope and sailcloth - forget it. Modern replicas almost invariably use synthetics, which look a lot different from what would be found on board a schooner of the 1920s or 1930s (like the Thebaud).
There's one other source you've gotta consult - if you haven't already. By whatever means necessary, get hold of a DVD or VHS copy of the movie "Captains Courageous" - the old original, with Spencer Tracy, Lionel Barrymore, and Freddie Bartholomew. It contains some of the most spectacular maritime footage ever filmed - all of it having to do with Gloucester fishing schooners of the 1930s. It's more than worth the trouble to sit through 90 minutes of melodrama (most of it only passingly similar to the original Kipling book) in order to watch the race between two of those wonderful ships near the end.
Hope this helps a little. Good luck. You picked a fine kit to break into sailing ship modeling.