I have not chimed in here yet since none of my Rushton's SLBC or Brown are 18 foot canoes. There were clearly differences not only between the 16 and 18 foot boats, but also between Rushton, Brown and SLBC. I do not own a Wells so I can't say much about those except that from what I have seen, there is the expected resemblance and enough commonality to make you think that parts were taken from the same source (thwarts, seats, etc.).
What is common between them is style of construction. There are always bow seat cleats. These are always attached the same way. The thwarts have the same distinctive shape, but on close inspection, you will find variation and especially in how refined the form is. The thwarts are never square cut on the ends, at least none of the ones I have seen. There should be two thwarts in an Indian Girl, unless it is a B grade. There is one thwart in an Indian.
A Whistle Wing will also have two thwarts as will a Brown.
Rushton seat frames all are virtually identical dimensionally and never have the coved rounding that is common on other builders (OT) seats. The stern seats vary between builders with SLBC and Brown using a rectangular shape and most (not all) Rushton's built with a trapezoidal shape. Some SLBC canoes are also found with a mildly trapezoidal stern seat. Overall length is not as important as the actual dimensions of the box.
Finally, WRT decks, these are where the builders made a statement and differentiated, and in part because Rushton allowed the buyer to customize. The typical Rushton deck is the short, lobed and somewhat clunky one. In the Indians, the deck is a simple coved shape although the all wood Indian may have a heart shaped deck. In an Indian Girl, the standard deck is the noted lobed deck. A king planked long deck was built by boxing in over the top of the standard deck. A standard canoe would be built and then converted in this case.
Long, single piece (and very elegant) cherry decks could be added to Indian Girls and were available (23 inch) on the Deluxe Whistle Wing. My Brown also has these decks. These long cherry decks, when present, elevate the canoe dramatically. You have one of these to use as a pattern. It will take some skill to duplicate. The coving should also be cherry and how it is secured is very specific. The American Beauty also includes both the higher sheer and these elegant decks. These (Beauty's) are pretty rare, at least from my experience.
Case in point, potato, potaaato, even though there are strong resemblances' between canoes built by these folks, there is also variability. When you restore one, you need to use whatever clues you have from the hull you are working on. Look for evidence of stern mounted seat cleats. Absent those, your stern seat will mount to the inside rail. Find varnish or paint clues inside the hull to show you where the seats were positioned if you do not have the holes present in the rails. Seat dimensions from a Whistle Wing or a Brown do not necessarily translate, but they will be c;lose. What a great project for us to follow. I'm looking forward to updates and wiling to offer dimensions whenever possible.