I temporarily mounted the 9" on the trailing arms. The top of the diff at full bump is no higher than the top of the frame rails, so that's good. I also ordered new axles with 5 on 5" pattern.....
There does not seem to be a dimension for where the mounts are to go on the axle; consensus seems to be "place them where they work best."
I picked up some 3/16" plate after all. After a fair bit of sitting and staring, I think I've come up with a notch I'm pleased with, though I may mock it up in cardboard entirely to see what I think.
Sharp corners are potential stress risers, so this is intended to reduce stresses. It also maintains a 4" tall frame through the notch as well. The front and back of the axle <em>cut</em> will use part of the original frame. It makes sense in my head.
Going to sleep on it before I bring out the plasma cutter.