you got it. A true SD40-2 has a longer frame. This is the big difference between the two. The other is pictured quite nicely in the front truck. SD40's, which were made between January 1966 and October 1972 were all equipped with the "Flexcoil" truck. There are two casting holes in each end of the thing, you see the front two here. The SD40-s however had come equipped with the HTC Truck and it was not only longer because all the traction motors faced the same direction (in a Flexcoil, two of them were back to back) but it had three casting holes on each end of the face of the sideframe.
The only railroad to abstain from the HTC truck because of in justified fears of derailments was Conrail. There dash 2's and SD50's came with HTC trucks.