my plan was to pratice by rebuilding a big hauler into some thing from the great northern railroad of Seattle and St. PaulPerhaps, as you have admitted to never having built anything before, and, indeed, seem to require full instructions on the basics of scratch-building as a modelling method, trying your hand at something a little less adventurous than a very large and complicated loco that never existed in North America might be a bettter plan.
Unfortunately, AFAIK, there is only ONE kit for building any kind of North American loco, and that's the Accucraft live-steam 'Ruby' - IF you can find one.
I fully realise that this is not something you might care to read, but I'm offering it as advice anyhow.
The many of us who scratch-build ALL started on something less advanced and over many YEARS gained experience by making use of other materials and methods. The incredible results from many scratch-builders are the products of much trial and error. As Mark Twain noted about whittling, the woodchips weigh can often ten times more than the finished toothpick.
The VERY best thing you can do is to find a group in your local area - some of them may have had the same ideas as you have. CapecodTod's model is an ideal one to emulate, IMO, and was the result of a LOT of hard and imaginative work and mucho head-scratching over the long time it took him to build it.