I would suggest that anyone who sets the NH ignition timing at 18mm before tdc and expects to get a 38 degree advance will be extremely disappointed.
The NH rod is 7” c/c
The rod stroke ratio does have an effect on ignition timing, on how hot the cam appears and on the torque the engine delivers. An alteration to the length of the conrod results in the piston being a different distance from tdc over normal ignition timing ranges. The displacement at top and bottom of the stroke 0 and 180 degrees is the same but the displacement at 38 btdc will not be the same if the conrod length is altered.
As a so called mechanic I have played around with different rod/stroke ratios ( and have adjusted the deck height accordingly) in the VH and was surprised at how much I had to reduce my ignition timing with the longer rod. I was also surprised that the cam appeared much hotter and had lost a lot of low down torque with the longer rod. Longer rod stroke ratios are often used in hot engines in vintage racing by those in the know. A longer conrod results in the piston travelling slower at the top and bottom of the stroke, ie there is a longer dwell time. The longer dwell time enables better high rev cylinder filling and reduced ignition timing. The 18.000 rpm formula 1 engines were running rod stroke ratios of around 2.8 to 1 whereas modern cars are closer to a 1.7 average, although modern high performance motorcycles are often around 2 to 1 or higher.
Ariel seem to run a rod stroke ratio of about 2 to 1 throughout the range.
This site gives an insight into the real world effect of conrod length. http://victorylibrary.com/mopar/rod-tech-c.htm