Yes, this engine has a long straight boiler, but it does seem smaller in diameter than the one you built, in relation to the drivers. If you could find a plastic pipe that would work with the smokebox from the big hauler boiler, that might be closer to the right diameter for this engine.
Doesn't the T-12 have a wagon top boiler? That wouldn't work for this engine.
As best I can measure, the large diameter part of the boiler on my Big Hauler 10 wheeler comes to 61 inches in 1/20.3 scale. If the boiler on # 4 was about 58 inches or so with the lagging, that's a pretty close match. It's pretty hard to argue about a 3 scale inch difference.
That would mean that the straight boiler that Pete made from a couple of donors is actually pretty darned close to correct diameter.
As for the domes, comparing the drawing to the engine picture, it looks like you would be able to use the sand dome for the steam dome, and find a smaller dome for the sand dome. The sand dome looked about the right size for the steam dome in the drawing. Just an idea for you.