The "wings" hide the motors which is nice. The parts match up with the interior of the Williams shell. Worst case scenario is that I'll split the pieces along the long axis and attach to right and left side of the inside of the shell.
The Lionel parts are a good starting point but I'll be checking my spares boxes for pieces to enhance the interior. There are a number of photo resources especially for the engineers side. I've seen a few pics of the fireman's side and it's definitely bare. I think all he got a set of signal light indicators and a seat. There was a walkway between the sides (through the arch). There was a set of tap switches that were part of the motor control in a cabinet behind the engineers station and a narrow walkway between the front and rear cabs along the outer skin. Center of the engine is where the main transformer (behind front cab) and steam generator (rear cab) were located. There was a lot of equipment hiding under that shell.
