Finally sorted out the drivetrain on the Holdsworth. The ring was on the inside of the spider, which was eating away at my soul. I fitted a shorter BB and also swapped the ring from a 3/32" 42T to a 1/8" 41T, which gives an exact 60" middle gear with the AM hub. You try finding a 1/8" 41T in 110 BCD!
Then I took out the threaded driver from the hub and popped out the bearing and dustcap ready for swapping to the 12-spline driver. Except...the 12-spline driver can't get as wide a chainline as the threaded one, so I gave up on that idea. I did swap the 3/32" 18T sprocket for a 1/8" EAI 18T. This isn't too hard to do once the driver is out of the hub. Put the bearing and dustcap back in and then put the driver back into the hub and adjusted it.
Next problem - the new 1/8" chain, a rather natty KMC K710SL, is only 100 links and I needed 102, Luckily, I had a bit of unused K710SL hanging around spare from the track bike. I'm sure that chain was longer, since it went around 52 x 17 on the track bike with links to spare.
The EAI sprocket sits a bit further out than the old Surly 3/32" sprocket, which helps with chainline but it runs super-close to the RH seatstay. If it ever actually rubs, I'll put a 1mm washer under both hub locknuts. This is my fault for reducing the AM hub width to the bare minimum (111mm!) because it looks cool.