Actually, the car was built by Pullman-Standard in 1957 for the NP. I don't recommend using the Rivarossi car because you'd have a hard time relocating the doors to allow enough room for the Vermonter mural (although that model it is basically correct for ex-UP baggage car 1852, one of the Ethan Allen cars). Also, for some reason Rivarossi molded the roof in the wrong curvature, but oh well.
I modeled 1800's sister car, 1801, by making styrene sides to go with the ends and cut-down roof from an Eastern Car Works core kit. You would get better results using the higher-quality Pullman core kit from Train Station Products. It's really quite easy to cut out the door openings from a sheet of styrene and then just assemble the pieces of the car together.
I had to eyeball the dimensions from photographs, but I've since drawn fairly accurate plans for the cars in HO scale. The drawings are posted on my
website, along with a detailed description of the project.
I hope you decide to take on the project - I'd love to model 1800 in that paint scheme, but I'm trying to stay focused on my early-'90s era.