Re: Building Sunshine Model kits
Over time, Martin Lofton of Sunshine Models produced a number of Mather kits appropriate for AC&Y. I think your kit is a very old one, one of the first kits Martin produced when he first got into the business, and Sunshine hasn't produced this kit for many years. Your kit (Kit 7.1) represented AC&Y's 600 series boxcars with a 7'8" interior height. The Life Like Proto 2000 Mather boxcar came along several years later, and is probably a better representation of the prototype. In its day, the Sunshine kit was the best available. Sunshine's kit 6.2 was released around the same time as 7.1, but it depicts the slightly taller cars with an 8'5" inside height. It has also been out of production for several years. It represents the 1100-1199 and 1250-1299 cars, and is the only truly accurate kit representing this exact variant. Nowadays, folks who build these 6.2 kits usually use the sides and ends, but they tend to use the underframe and roof from a Proto 2000 boxcar or stock car because it's easier and the detail is better. AC&YHS member Mont Switzer published an article in RMC or Mainline Modeler several years ago, explaining this technique. The Proto 2000 kits provide correct trucks, roof, underbody, and detail parts, so it's worthwhile to do this. Martin also produced a Mather stock car in both double- and single-deck versions. These have also been out of production for many years, and the Proto 2000 version is better and easier to build anyway.
When I first read your post, I thought you were talking about the Sunshine kit 103.4, which represents the AC&Y 3000 series of 10'3" tall cars, and which is currently available. These kits are much more state of the art, and much easier to build, although they certainly don't just fall together. Many of the more difficult steps required for the early cars, have been eliminated in the newer 103 series kits.
The older kits like yours are true flat kits. It's not uncommon to find some warpage of the sides. This can be taken care of by applying very low heat with hot water or a well-controlled oven to let the parts flatten out. You'll notice that the roof is molded flat, with no peak. The idea is to build the box (sides and ends) and use the long square piece as a ridgepole. The flat roof pieces are then cut apart and placed over the ridgepole. The use of the rectangular piece of styrene isn't specifically explained. In the instructions, step 3 says to cut that piece so that it exactly matches the underbody casting in its final shape. I interpret step 5 to mean that the styrene rectangle is the subroof to which the ridgepole is attached. This keeps the carbody square by ensuring that the subroof exactly matches the size, shape, and corner angles of the underbody.
Among the papers should be a four page item labeled "Instructions; Mather Boxcars". If you don't have that, contact me at firstname.lastname@example.org, and I'll copy mine and send it to you.
If this is your first resin kit, I recommend buying a more state of the art kit to start on. Build one of Westerfield's fine kits with a one-piece body first, then graduate to a more up-to-date flat kit like the current Sunshine 103.4 kit. After that, you'll be better prepared to tackle the early Sunshine car.
AC&Y's Mather boxcars generally used Klasing power brake systems. The Proto 2000 Mather kits include an early version of the Klasing gear, which can be another reason to get a Proto 2000 kit as a parts source. Sunshine's kits invariably include an Ajax power brake because the correct parts are not available as detail parts. I build my Mathers up to the point of installation of the power brake & wheel & stop there, waiting for the day somebody produces this Klasing part. Contact has been made with potential manufacturers, but nobody has stepped up to the plate yet. Builders of Sunshine and other resin kits should also look at the aftermarket products of other companies to see about ladders & similar fixtures that might be better than those in the kits.
Resin kits can be a challenge, but the results can be quite rewarding. Don't be discouraged.