They don't have a detailed parts list for the Trainman 68' Flat Car, but here is the diagram for the 62' Bulkhead car. The body on my bulkhead cars looks to be the same body as the 68' Flat car, but the bulkheads reduce the interior length to 62'.
https://shop.atlasrr.com/parts...lat%202-3%20Rail.pdf
In the parts diagram, the 3R trucks for this car have a different number than the separate sale trucks Atlas offers. When I compare the couplers on my cars to medium and long shank couplers that are available for sale, the medium couplers are too short and the long couplers are too long. Only Atlas could tell you if the truck identified in parts diagram is available.
Attached is a picture of the end of one of my 3R flat cars. The entire end of the car is open so the coupler can swing. Your 2R version looks like there is a piece of plastic preventing that. Also remember that even in the 3R version this car requires O54 curves or wider.