I was able to get back into the workshop long enough to button up the jackshaft assembly. I shortened the keyed shaft, then faced it and cut a channel for the circlip in the lathe.
I made the last remaining spacer needed and assembled it all. I was a little bit generous on the shaft length, because I wanted to ensure I could align the sprockets correctly.
I installed the primary chain, and my adjusters worked perfectly (though they’re a real bear to get in and adjust). Here’s the chain at the most slack setting:
And here the jackshaft has been moved to the third position, out of four:
You can see there’s about a 1/4” extra shaft sticking out on primary side, but everything spun smoothly and easily.