Not a bad start, but probably worth pointing out that the GP7/9 is considerably longer than the U18b. The Alco RS3 has the exact same distance between center bolsters as the U18b, so I would advise revising this model to fit the Atlas RS3 chassis. (which is also cheaper, and has AAR B trucks)
An alternative option would be buy an Atlas U23b, and then modify the shell and fuel tank to fit on the Atlas RS3 chassis. You could then sell the U23b chassis on ebay. Just a thought.