Good progress today. Match-drilled most of the rest of the holes needed on the left horizontal stabilizer. It's pretty easy - just have to make sure everything is aligned properly and watch edge distances.
One more hour of work on this and we should be done on the left HS. Rinse and repeat for the right side - then clean all parts up (deburr, edge finishing, etc) and rivet!
You can see my progress in the photos below.
Clamping and fitting the HS-405 and HS-404 ribs carefully