Made good progress today. Assuming I don't run into any issues, I may have this done tomorrow!
The advice of moving slow is invaluable. So far I've only run into silly problems.
I forgot to put in the fuse, which took me longer than I want to admit to figure out. I knew something was off when I wasn't getting voltage from the switch. On the upside, there isn't too much to check when there's only a few wires hooked up.
The other thing was only one half of the 12au7 was lighting up. I resoldered the A4-A5 connection, and that took care of it.
So, all in all, a pretty good day.