I think it is safe to say that the chapel cars were all standard gauge, at least the ones described in the book by Wilma and Norman Taylor, This Train is Bound for Glory: The Story of America’s Chapel Cars. If there were others, I don’t know about them. The cars were built either by Pullman or Barney & Smith and the rail lines gave free passage and built sidings for them to remain free of charge.
The best place to start is the Taylor book. It contains many pictures of the chapel cars and has detailed floor plans for each of them. From the pictures the rest of the mechanical details could be pieced together. I haven’t read the book for almost ten years so I need to read it again, and soon!