Bwanabob, one thing no one has mentioned is that the light fixtures in these cars are held in place by a clip that is molded into the body on the bottom of the interior. So to put interiors into these cars, would require cutting those off and using some sort of overhead lighting, like what K-Line used in their 13-1/2 inch long streamliner cars... later also produced by Lionel under the K-Line By Lionel banner and then by RMT (Ready Made Trains).
I think I've seen mention here of circuit boards with this overhead passenger car lighting via LED's being available from someone, but it sounds to me like - unless you want the satisfaction of the project - the K-Line/K-L by Lionel/RMT type of cars might fit your bill as they have interiors and though a little longer than the Lionel streamline car you have, they are the same width and height. The recently tooled Lionel and then the MTH Railking streamlined cars are a little bigger in size, but that depends on your tastes.
Actually I should note, not all of the K-Line steamliners had interiors. That was a feature K-Line added later on to these cars. Make sure the description states interior detail.
Also if you're using 027 track and switches, I've heard a few observations, that these cars can potentially rub against the 027 switch box housing. But then others seem to not have that issue. But since tubular track does have some play in it as far as trying to make something fit by bending the curve a little tighter - this could be the source of that issue.
Another idea for you: I had a friend, who from someplace, found color photos done of passenger car interiors shot from the outside through the window. He took those, scanned them into his computer, and then resized them to fit into the Lionel 027 passenger cars you describe. He put clear window glazing into the cars, the printed window inserts next with a piece of thin plexiglass behind that to help distribute the light more evenly though the windows. So that's a possible approach.