Bob: Now that you show the photo of what you want to do, I can see your problem. That sure is an odd car in the shot; probably done for some specific commodity by the RR itself.
I'm not sure just what you can do with those cast in posts, if you want to have those doors open. And, actually I'm not sure that the door openings would be correct anyway; The standard door, as on the ACL car, is centered in the side of the car. On this unusual example, the actual center of the door looks to be to the right of both doors. I thnk that you might have a bigger project there than you realize, actually cutting out some of the car sides and moving them to new locations. And then there remains the problem of mounting the body to the floor. I think that I would start with a regular 4-door automobile boxcar and see if the doors could be moved to one end.
Be interesting to see what and how you actually do the project. Sorry I can't be of more help.
Paul Fischer