Oboy, talk about MEMORIES!! I built an Ambroid kit many years ago when I was in the Air Force. It was the 3-in-1 passenger car set (I believe Boston and Maine open-vestibule prototypes), and they were not only difficult, but as I remember, one of the most satisfying wood kits I ever built. It took me several months to build a baggage, combine and coach, and I don't even want to tell you how many X-acto blades I went through.
I wish I could remember enough about the kits to tell you what I did, but I remember it involved a lot of very fine grit sandpaper, TONS of blades, extreme patience, the then popular Ambroid Wood Cement (which was like iron--it REALLY held!), and following the directions to the absolute letter. No short-cuts at all.
I ended up using Kadee #5 couplers and at that time (new on the market, BTW), Central Valley wood-frame sprung passenger trucks. As I remember, the worst part of the kit for me was installing the underbody brake mechanisms and the open-vestibule railings onto the metal castings--I still don't know whether they were pot-metal or zinc--and getting the roof contours right with blades and sandpaper. When I was finished, I was absolutely exhausted, but they looked pretty darned good. I still have two of them stuck away in a closet, somewhere. I'll have to get them out and see if they looks as good now as they did when I was in my 'twenties, LOL!
They are Absolute Craftsman kits, make no bones about it. But as I remember, the directions were very well done and very specific, and only worked if you followed them to the letter.
I have no idea if the Ambroid Cement is still available, but if it is, I'd get some. It was the best wood cement I've ever come across.
I would assume that whatever Ambroid kit you decide to build, you'll need additional weight inside the car body before you complete the car. They were, as I remember, very light kits when finished.
I know this isn't too much help, but you sure brought back some neat memories! Back then, as far as wood kits were concerned Ambroid was to Silver Streak like current Tichy is to Athearn BB as far as difficulty.
But boy, howdy, was the result ever WORTH it! Have fun with that car. Northern Pacific used them when they were building across the plains in the 1880's. It'll be a real conversation piece no matter WHAT era!