Here is a very brief description of how I cut the cars:
1. Figure out how much you want to cut out of the length. It is best to have a general measurement, say around 10 scale feet, not 10 feet 3 inches. Look at the model to see where the cuts can be made, taking into account the stake pocket spacing, floorboards, etc. Mark these cut areas with a Sharpie.
2. Disassemble the model. Separate the body from the chassis and the floor casting.
3. Cut the body and glue back together using a straight edge to line up the pieces, don't depend on the accuracy of the cuts. Allow this to dry a couple of hours.
4. Fit the body bolsters into the body. #901 I moved them outward from the stock model and #902 I used the stock location. Cut up the metal chassis to fit inside the body, with the bolsters properly located.
5. Mount the body mount couplers, add details, paint, lettering, etc.