I sent a note to a friend who is the GCR shop foreman about the differences, in addition to the steps to the running board and the piping on the fireman's side. Here is his detailed response:
Loco has Stephenson (inside frame) valve gear and piston valves. 29 was converted at LS&I to Baker valve gear (outside frame).
The front steps – as you previously mentioned.
Running boards – 29 had an air compressor mounted on the engineer’s side, so the running board goes up and over this appliance. Same for the fireman’s side for a Worthington BL Water Pump (which I can’t see on the model)
29 has top boiler checks, not side. Also a sander line missing to the front of the #2 driver
Lifting injector on Engineer’s side instead of non-lifting.
Tender trucks should be standard AAR roller bearing freight trucks.
The large radiator piping on the sides was a Franzen (prior Supervisor) addition. They are for cooling the compressed air, and also for hiding the air reservoir.
But I plan to scratch build the steps and the piping and I can live with the rest of the differences, BUT only if the passenger cars are Harrimans! Otherwise, will have to pass.