Update: I was thinking of adding something to tie into the bell housing as Dylan suggested, though I don't think I can fit anything there without interfering with the axle or down-pipe.
Not a huge deal really though. I'll go ahead and test the reinforcement as-is when the stronger bolts finally come in to see if it helps. My measurements along the top of the case didn't show much movement, whereas the bottom kind of twisted out toward the passenger side of the car by a couple of thousandths of an inch (torsional distortion). The nice thing is that most of the twist was registered behind the front diff where the case starts to narrow down into the compartment that holds gears 1 through 5... but we'll see how it goes.
On a bright note, I modified the test fixture to clear the diff brace and made another front diff brace for testing. I found the dial gauge only barely registering .001" of expansion just above the driver's side axle stub with it installed where I was seeing a hair over .003" of expansion without... So I'm happy to say that the diff brace definitely is making a difference