On the subject of cylinders and steam pipes, I've had another look at the drawing in the Haresnape book I referenced earlier and compared it to the one for the Nelson. It is essentially a 4-8-0 Nelson, with 4 cylinders of the same size and the outside steam pipes towards the back of the smokebox and outside cylinders. The outside cylinders are also slightly angled as shown in the painting. This is the arrangement for the original cylinders. I do not know what the arrangement for the Bulleid cylinders is , and it would be a matter of speculation as to whether the 4-8-0 would have received Bulleid cylinders. However, it is 3 inches longer overall than a Nelson and the boiler pressure stated is 200 psi rather than the 220 psi for the Nelson. Although the drawings are small, not to the same scale. and not fully dimensioned, I would hazard a guess that the smokebox on the 4-8-0 is a bit longer, the extra length being between the chimney and tubeplate. This could be to keep the cab, blastpipe, etc. in the same relative positions because of a longer chassis.
According to the text, the 4-8-0 was a 1935 proposal for a standard heavy goods, although the notes for the drawing say it was for Kent coal traffic. However, the length of sidings and running loops on the main lines limited the length of trains to loads that could be handled by the S15, and so the idea was dropped.
Best Regards
Geoff