Those results are impressive. I have one of these Williams that I detailed a la 700E, and in which I installed a Cruise Commander/pre-RS Sound Commander. This made it useable. I have been happy with the now-steady running speed and slow-speed performance - at least until seeing yours. Gee, thanks.
I've posted this photo of it before; the valve gear still needs to be corrected/completed. That sand dome is a separate piece, as I recall - I was tempted to do a square sandbox/B&A J2 treatment.