A bit of an update. The axle nuts I ordered turned out to be out of stock. These seem to be a bit scarce or command stupid prices. Instead I bought a pack of stainless M10, 1 mm pitch nuts. Bored a couple of them out to 9.5mm and then screw cut them in the lathe to 26 tpi, worked perfectly.
The conversion is nearing completion, just need to do the cabling and the disc brake is going to have to wait. I have come to the conclusion that dual drive conversion is better done on the D9 rather than the I4. This is because the I4 requires more parts, a rear deraillier, two gear levers, potentially a larger chain ring. The I4 is fitted with a 44 tooth so may be under geared and due to the angle of the chain run the chain is likely to slap the frame at the rear. A larger chain ring should increase the angle and move the chain away from the frame. Also the derailler hanger on the I4 has a step in it to accommodate the chain tensioner but this prevents the derailer from reaching the lowest gear. I had to mill 4mm off the step to get enough derailer travel. See photo.
I am sure when its finished I will be very happy but I think a much easier and probably cheaper upgrade for the I4 is an internal gear hub with many more speeds such as an Alfine 8 or 11. I would also seriously consider a front derailier and multiple chain ring conversion on and I4.