Please check the body casting.
If it looks like this photo;
With no shelf at the end for the floor and the notches for the crossties and bolster close to the edge of the side sill,
You have a bad body casting!
There are only 4-5 of them out there like this, but I need you to contact me so I can send you a correct body casting.
Email me at firstname.lastname@example.org and we can discuss getting you the right body casting .