I have 3 or 4 OK Engines shells I'm using. I made floors and ends cut from sheet acrylic plastic. The vestibule end has the inner wall glued in place (as is the opposite end on the car) but the actual end cap is a slip (firm) fit. The opposite end has 2 screws to keep the floor from moving, once these screws are removed and the vestibule end is slipped off, the floor can be slid out.
There should be a number of photos on the forum, search on OK Engines or Silver Meteor.
Now that I have the GGD Silver Meteor set I may go back and revisit my old cars and do some upgrades. I think I used some GGD trucks as well as Lionel (K-Line) trucks when I built mine.