Pete,
Yes I had noted that your dad had built the engine.
I might need to modify the attachment for the additional stretchers because my cylinders are already machined. I had decided (25 years ago) to put many more fixings on items that can be 'seen' such as the cylinders, buffer beams and saddle. For example, there are actually 8 bolts per side holding the real saddle to the engine frames and this is easy enough to copy.
Your picture has helped me to clarify in my mind how to balance the detail versus the functional model. I have decided to make the inside of the frames very practical i.e., as the model design e.g., use out of scale 3/32" rivets to hold the main horns and buffer beam angles together for construction speed and because they are out of view but try to copy some of the simple external features of the real thing e.g., the buffer stocks and heads and various other details.
Phil H