Well, preliminary verdict is that this works, and it's safe.
I cut the heads off of the factory bolts to salvage the spacers and drill them out to fit the 12.9 grade bolts.
I ended up using a 5/16" carbide bit, with a second person dripping cutting oil around the bit, to drill them out. The spacers are hardened, so beware. They will take some work to ream out to the right size.
That's the final setup. I torqued them to 35 ft-lbs, and used a bit of anti-seize between the spacer and the bolt, to inhibit corrosion. I did not use anti-seize on the threads.
The only issue I seem to have is a little bit of groaning from the left front, but I think it's because I need to re-align the struts in the towers, which I'll do at work tomorrow.