Alright, thanks for the help all, I finally succeeded! I had to order a keyseat cutter, so that took a few days to show up. I got a 5/32"x5/8" woodruff key. 5/32 because most of the other keys in the lathe are 5/32, and 5/8 instead of 7/8 because that was a touch shorter than the length of the gear. It allowed me to a use a cheaply made collar instead of using up one of the keyed ones.
Assembly was a bit frightening, lining up the keyway was very difficult since there is only about a 50-tho area where you can see the key not yet on the gear. After about 20 half-assemble/disassemble cycles getting the leadscrew into the gearbox, it 'just worked' once! I tightened down the 1/2-20 nut on the end with my shaft collar in between, and locked it all down. The rest of attaching it to the lathe went easy enough, though the auto-crossfeed gear was a total pain to get lined up right.
I put on the banjo (which I'd taken 1/4" off of the thickness, otherwise the gears didn't line up right), popped the 48 gear tooth on the gearbox and 24 on the stud gear. I used a 60 tooth in the middle (since that was what is on my adapter collar) and bolted it all together! I did a 14 tooth run and checked it with my thread gauges, and it was dead on!
I've got a new door coming soon, so I think thats my only thing left as a 'TODO' before I call this project a complete success!